The image on the right is an exerpt from the QHJ, early 60's, painting a stark picture about the need to take extra care.

  The opinions that I am presenting below are far from a comprehensive treatment of the subject,  just some of the conclusions that I have come to over the years. Another fellow has posted on the subject, and I think he has done a pretty good job. I believe that the hitch and ball should be lubricated with grease, rather than oil, which he prefers, but in the main we agree: lubricate the hitch and ball.

Much this is kinda basic, intended for people relatively new to hauling, so pardon my covering some old ground. 


  In General  I have heard that the harsh ride (trailers need stiff springs and have no shock absorbers) and lack of circulation in the feet (somewhat dependent on the animal moving around) can result in laminitis. I don't know what the safe limits of travel time are, I suppose it varies, depending on the horse, the road, the trailer, and so on. I don't know anyone who has had this problem, so I can't talk "case histories". 
  We  have always bedded heavily with shavings, not only to prevent the floor from becoming slippery, which could cause a fall, but also to cushion a little, and stop to offer water not less than every four hours. Many people carry their own water, as you never really know the quality of what is available on the way. (Many plastics, and all black rubber containers impart a flavor to the water, which the animals usually don't like.) The way I heard it is that Poco Lena's owner's airplane was reported missing, and she was forgotten in a trailer for a long time in the resulting excitement and confusion. The possibly exaggerated story told me was that her hooves actually fell off. It is a fact that she spent her few remaining years lying down almost all the time, and was ultimately euthanized because of her suffering.
We expect to reach our destinations without horrible medical emergencies, but just in case, a fairly comprehensive medical kit is a good precaution. In event of the worst you will probably have to rely on your own capabilities. Most things are obvious - lots of vetwrap and bandaging supplies, betadine surgical scrub, jug of distilled water, clean bucket, roll of sterile cotton, etcetera. 
But - depending on the location and nature of injuries, it may not be possible to stop serious bleeding with pressure, or hold everything together with bandages. I am not convinced that the "Sportsman's Guide" always offers the best deals, but I obtained a simple surgical kit (unused army surplus) from them for less than $40. (Do not give them your phone number - they will sell it to telemarketers.) This kit does not contain enough suture. I use Valley Vet, but imagine that there are many similar sources for things like that. Valley Vet has surgical supplies at tolerable prices - we have done business with them many times without any problems. (The case, incidentally, comes from Harbor Freight for less than $20, when on sale.) In event of treating serious injury at the roadside, most tranquilizers simply will not work, and you may need two cotton ropes in not less than a one inch diameter, and at least fifteen feet long each. You will need to know how to lay the animal down and immobilize it without causing additional injury. That really calls for illustrations, and I begin to see that I will have to elaborate on this and make a separate page for it. That will have to wait, and there are books and articles that deal with that subject, although I do not recall any specifically.

About trailers themselves, I have some fairly definite opinions, which may (or may not) be useful to you.

Chains   Trailers do sometimes uncouple from tow vehicles, so the law requires "safety" chains. These are usually so weak, and further, attached in such a manner at both ends, that you might as well use a rotten shoelace. Use two 3/8ths inch chains, criss-crossed under the hitch (assuming this is bumper pull) to form a cradle of sorts, to catch the hitch if it falls. Cut them as short as will allow free motion between trailer and tow vehicle when at their most extreme angle to one another (backing up). If too long, they will allow the trailer to beat back and forth more violently, making your vehicle more difficult to control. Also, excess length may allow them to drag at times on the pavement. Chain should be a rated grade, I forget what, and be aware that if welded directly to the tongue, it's temper has been destroyed adjacent to weld. Its strength is greatly reduced at that point. Needless to say, choice in hardware at the "truck end" is critical. I would evaluate very critically how much punishment what they connect to can endure. Most bumpers and/or receivers are somewhat minimal - in structure, in mounting to the frame, and the chain hookups. The standard behind mass production can usually be summarized as "engineering minimum". I've never heard of anyone having trouble with bumpers coming off, but I would give every part a very long hard look. (Mounting bolts too, how many, how big, what grade?) A little extra strength won't hurt.
Breakaway kit    A  little chain or cable ("B") connects to one of the safety chains with just enough less slack that if the safety chain is stretched straight it will yank the little one, activating a switch that sends power from a small battery carried in the trailer to the trailer brakes. Harbor Freight might be the cheapest source for a meter to get an accurate reading of the electrical charge remaining in that battery. That must be checked regularly.
 It's all fairly obvious, good clean connections at battery and switch, proper adjustment of the activating cable / chain. An improvement that I have considered, although not tried yet, is another variable resistor inline from that battery, adjusted so that the brakes will come on good and hard, but not lock up.
Ball   For a two-horse, the ball is preferably cold-forged, on a one-inch shank, 7500lb rating (minimum). J.C. Whitney used to have that sort (among others) in their catalogs, and may offer them online, too. You can sometimes find a 2" ball on an 1&1/4" shank, so much the better, but I would keep away from anything that does not have it's rating (in pounds, not category) stamped on the top of the ball as on the right. 

The solid stainless steel are sexy, but can't compare in strength with the cold-forged high carbon kind, as on the right.  Always grease the ball before you hook up, and clean inside the hitch (old rag) to remove abrasives that will collect in there. It doesn't take that much wear to make a combo that will pop apart under just the right (or is that "wrong") conditions.

Wheel alignment   Picture the outer edge of each tire as a vertical plane; down on hands and knees, sight along the sides of the tires to see whether the plain represented by fronts is a fairly close match for the rears. If you line your eye up just right, any significant difference will be quite obvious. It could be a bent rim, but that can be determined by jacking it up and spinning the wheel. It is surprised how many people have really bent axles that they never even noticed. (Once you've kind of trained your eye, these things just jump out at you.) This is only significant at highway speeds, and even then, may never be a real problem. Any decent welding shop can easily adjust this. Likewise, a sufficiently skilled "handy" type person needs only raise that side, heat the axle evenly, inboard of the near spring, not too much, no color, and no significant heating of spring or shackles. Apply a soaking wet rag to the area that you want to have shrink disproportionately to the rest. This will warp the axle: Done right, corrects the problem perfectly, done wrong, makes it worse.

Wheel-bearing adjustment   Many trailers have really loose bearings. Sometimes there are differences in the design and adjustment of trailer bearings compared to cars, although Chilton's would likely be a good place to get some general ideas. I don't consider this a completely reliable method, better to jack it up, but between service intervals I check for excessive looseness with this quick and dirty. Putting a knee against the top of each tire and push repeatedly, hard enough to make the entire trailer rock back and forth rhythmically. If that bearing is seriously loose, the wheel may make a sort of thunking noise, and you will very definitely feel the looseness. A little (very little) is normal and needed - out on the road they will warm up, making the fit tighter. So, a word of WARNING! An over-tightened bearing will overheat, very possibly lock up, can ruin the spindle, and rarely, set the grease on fire. Don't go there.

Still, some spindle/bearing designs seem to offer only two choices, too tight or very loose. In one case it bugged me so much I got a shim with a small interior diameter and spent forever grinding/filing out the inside so as to provide a little "tab" to latch into the groove on the spindle - and years later discovered that some trailer supply places have nuts with twice as many slots cut in the top, providing twice as many increments of adjustment than the original nut. Live and learn.

Anyway, if it has to be too tight or too loose, choose loose, it might reduce bearing life somewhat, but I would be concerned that too tight could cause drastic problems. Between those "aftermarket" nuts and the fact that the washers intended for this use are sometimes thicker or thinner, it is possible to get into a "reasonable" zone.

Incidentally, replacement bearings and seals for discontinued, or even unknown wheels are available at places that specialize in bearings, just check the yellow pages. Bring old parts along, as they will probably replace by size, rather than by make and model. 
Brakes   My personal preference strongly favors the hydraulic-activated control units. These send electricity to the brakes in proportion to the pressure in the tow vehicle's brake system, so there is the potential for perfectly synchronous braking action if everything is properly tuned. "1" is a stack of seven or eight leaf springs which retract the hydraulic solenoid (piston) contained in the casting marked "5". They also counteract the pressure of the brake fluid variably, according to the position of the white supporting block, which is moved by turning the knob "2". As you can see, moving it closer to the solenoid will make the stack of springs stiffer in relation to the force exerted by the solenoid.
The image on the right shows detail of the contacts ("3") inside the unit. Again a stack of leaf springs, in this case they  conduct electricity. Each "finger" touches a separate contact when depressed. Each contact is connected to a separate resistance coil "4" - there are a total of seven seven "fingers" and seven related coils in this particular unit. As each additional coil receives power, that much more is sent to the brakes.
These contacts are made sequentially, relative to how  hard the tow vehicle's brakes are applied, and therefore how far solenoid is able to press them down. (Naturally the longest fingers touch their contacts first, the shortest last.) One of the truly ingenious elements of this device is that each finger presses down at an angle, so as they are pressed farther down beyond initial contact, they slide along those surfaces. This makes the contacts self cleaning!

With the trailer empty, on dirt, at maybe three miles per hour, have someone watch what happens as you apply the trailer brakes progressively, to the point that they begin to lock up. Does one wheel in particular always lock up considerably before the others do? If so, this is a likely sign of future trouble. (This assumes that the tires are all the same.) Jack up that side, and you will probably find that that wheel will not spin as easily as the other. Insufficient lubrication, or bearing too tight are likely, possibly badly adjusted brake, but whatever the cause, resolve it. Conversely, if all the others lock up, and one never does, this also needs to be looked at. Again, with the trailer empty, and again, at low speed, but this time on pavement, apply the vehicle brakes reasonably hard.
If the trailer brakes lock up really excessively, even when you adjust the unit under the dash, you might consider getting a separate variable resistor. This is placed inline into the feed to the trailer brakes, and is normally mounted on the inner fender under the hood.
 You can adjust this according to the load in the trailer, for the maximum braking force just short of the point where you start having a problem with lockup. (It is a simple resistance coil, bridging it closer to the input/output end sends more power to the brakes.) If they tend to grab after some use, it may be that they are adjusted a little too tight, (the brakes themselves, as opposed to the bearings, separate issue) or they may have cheap linings, which get really hot, and transmit too much heat to everything around them. It used to be that "semi-metallic" friction materials were the ones of choice, although now severe duty materials are called "carbon metallic". I don't know what (if any) difference there is between those two. Anyway, I have had many people try to sell me the lower grades (for cars) with earnest and sincere assurances that the inferior product will satisfy. That is totally incorrect, so beware.
On the right - A fairly typical electric unit. This assembly bolts to the axle, and the spindle protrudes through the hole in the middle. "1" is the electromagnet, which becomes increasingly powerful as the electrical current passing through it is increased. It (magnetically) grips the inner face of the wheel drum which turns around the stationary parts at right. The magnet is mounted on a moveable arm, "2", which pivots on on a large stud, "3". A cam of sorts, "4", is also mounted on the arm, (2) which levers the two brake shoes, "B" outward against the inner circumference of the drum. 
On the right - The arm has been pivoted to the right, showing how the cam (4) levers the brake shoes apart. The arm can move in either direction, with the same effect on the shoes. Spreading them at the top in that manner presses both shoes against the drum pretty much equally because although they center on the same dowel the retracting springs are hooked on, (just above "4") and so cannot rotate, they can "float" somewhat, which makes them more or less self-center within the drum when forced against it.
Since the magnet is in firm contact with the spinning drum any time power is fed to it, it can get very hot, and burn the wires. These can be glimpsed in the photos above, emerging from behind the magnet, going up, and then around behind the arm. They are fastened to the arm until they get near it's pivot, so as to pass out the back of the base plate at a point where they aren't required to move much. If replacing these, normally magnet and all, special care is needed to first fasten the wires securely to the arm, (a little more than factory can't hurt, might help) and then to provide enough slack to ensure free movement between the last fastener and the point they exit the base plate without leaving enough slack to permit interference with other parts.

 Latches  If it is the sort with recessed latches, with the paddle-type handle on feed, escape, and tack doors, take a close look for evidence of how far the latching tabs actually insert into the holes that they snap into. Sometimes it isn't far enough to prevent doors from popping open. I have had trouble especially the tack compartment doors, which seem prone to working open as you travel, but provided there is enough engagement of the latching tab, keeping them locked will prevent this. If the locks don't work, or the keys are lost, I turned a recent lock replacement into a "how-to". Some years ago a girl from Washington told me about someone who dumped some cattle on the road when the rear door inexplicably popped open. One of ours has a rear door that is fastened by hooks that rotate around and engage the door. Awhile ago I noticed that two of the three hooks were gradually bending, and not latching in nearly as far as they originally did. I suddenly had a theory about what happened to the fellow in Washington.

 Tires   You may notice that the tires look well inflated until the horses get in, whereupon, one or some, suddenly look low on air. Some just lose more than others do. The inflation gauges at gas stations are notoriously inaccurate, and a good one of your own ought not cost more than ten dollars. Always check them before you leave home. Particularly if using a heavy-duty tire (a good idea) you may notice as time goes by that the tread is wearing a lot more in the middle than at the edges. This indicates that they need not be inflated to the maximum pressures indicated on the sidewall, but reduce it judiciously, and check that they aren't hot after you've been on the highway fully loaded. (Always inflate according to the load.) Another thing is that often, tires destined to go on trailers are not balanced, as would be the case with a car. It may be that they were, but look for the little lead weights clipped on the rims. Your horses will like it better if there are some.

I assume that everyone is acquainted with the usual brainwashing tactics, such as routinely feeding them grain in the trailer, to make them like it in there before going anywhere. The last thing you need is an animal going nuts in the trailer.