When it comes to horror stories about untied horses getting out of the trailer....by the same token I have also known of a couple instances where a horse was very badly injured by a trailer door coming open, and the horse ended up out (or partly out) of the trailer while still tied inside—these were full size horses in a standard front facing trailer—they were tied but when the door came open they either stepped back or fell back and their hind feet ended up out on the road while their heads were still tied to the front of the stall—not at all a pretty picture by the time the driver got the rig stopped, because once the horse's hind feet are out of the trailer, which is travelling at speed down the highway, he hasn't a hope of getting back in again. He can only drag. People suggested that the horses would possibly have fared better if they had been able to fall right out of the trailer rather than being dragged…. The same thing could happen with a Mini tied in a box stall, if the stall is right at the back of the trailer (or the horse goes out an open side door). Tying does not guarantee safety, and in some situations may be more unsafe than leaving the horse loose.
Janiecakes--I haul a 14 ft stock trailer that is 5' wide. There is a center (roughly center) gate so if I am only hauling a couple of horses I put one in front, one in back (or both in front if they are buddies) and leave them loose--they have a box stall that is roughly 7' x 5'--the front part is slightly longer than the back part. They generally ride facing backwards--I drive carefully so they aren't going to get thrown around in the normal course of travel. If I have to brake hard to avoid something then they would get skittered to the front of the stall--but I feel that is safer than if they are tied & the same thing happens. In the case of a major wreck, it isn't good either way--at least loose they will not have their necks broken by the tie rope--and if the trailer were to flip or roll, a tie rope wouldn't help them at all. And believe me, I drive defensively to try and avoid any such thing happening!!! If I'm hauling 6, as I often do, then I tie, three facing forward, 3 facing backward--simply because I'm usually hauling at least one stallion and not all of the others really know each other--tied, none of them can cause any trouble for any of the others. Loose it would be a problem in some cases.
Before getting the stock trailer I pulled a 2 horse straight load trailer with full divider. I would either put one horse on each side and leave them loose--they would generally turn around & face backward--or I would tie the divider to one side & then I could haul 2 or 3 in the big part of the trailer--again, left loose.