We pulled an 18' Cornpro 3H BP stock/horse combo for over a decade. It had a ~5' dressing room ahead of a moveable forward bulkhead.

With three horse stalls, we would trailer a single horse in the center stall, two horse haulings were split between the front and rear stalls. At no time could we haul a single horse in the rear stall, without unloading the hitch. We hauled this way while using two trucks and a SUV, and by load shifting according to the number of horses, we successfully kept the hitch weights at a minimum, whilst maintaining proper pulling characteristics.

If you were to add enough weight to the front stall, (I assume you have a three stall trailer), you would be able to haul a horse in the rear most stall. There is a way to determine the amount of weight you would need in the front. Measure the tongue weight of the empty trailer on the hitch. Load a horse in the rear stall and again measure the tongue weight. The difference will be what you have to add to the front to maintain a towable trailer. If you have a supply of sand or feed bags, you can add them until you again have the original tongue weight

One thing about our Cornpro. With our trailer, the tongue would not touch the ground if the front jack was retracted when it was unhitched. At this point there was a zero tongue weight as the trailer was basically balanced on both axles. When the tongue was lifted enough to engage the hitch ball, the weight rapidly increased to ~ 650 lbs as the height increased. With this weight and height, the trailer would tow perfectly when empty. With a single horse in the center stall, which was slightly ahead of the center line of the two axles, the hitch weight would increase ~100- 150 lbs. Again the towing characteristics were very competent.

Quite a bit of this discovery can be made whilst the trailer and truck are stationary. Once you have established a necessary weight addition, you should end up towing a stable trailer with a horse in the rear stall..