I had one who did this. Turns out she had arthritis in her hocks which made balancing painful. Hock injections and pain meds helped. But what worked best (in addition to those things) was moving the divider over to the side and giving her the whole trailer space to balance as she saw fit. That stopped the butt leaning and made her a much less grumpy hauler.
@whitmers was yours a slant or straight load? I could take the divider out for her, but then there would be no butt bar or anything to keep her in for unloadingāI usually haul aloneā¦
I like the slant load trailers. How do you think she would haul in one of those?
You can usually get or have fabricated a butt bar for the whole width of the back of the trailer.
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I have a 17.2 mare that also likes to lean back on whatever she can so I sympathize greatly. Mid pandemic I got the chance to buy a feather lite 2 horse BP (mine has no ramp but they do make it with one) and I jumped at it. Only problem- no butt bar, instead it has butt straps. I originally was going to tear them out and get butt bars installed but after doing research realized these straps had two benefits. First of all, my leaner can lean all she wants end it wonāt warp because itās fabric and secondly, the straps have a safety feature the bars donāt- if you need to, you can cut them to get a horse out unlike a bar which can wedge in an accident god forbid.
So it was a good all around solution to the leaning issue and has an extra safety measure.
(For the people who say the butt bar can give the trailer stability in an accident, all the research I read about it basically said if your trailer is so fragile the butt bar is part of whatās keeping it solid in an accident, you donāt want a horse in itā¦made sense to me)
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@Cocorona good to know! @moonlitoaksranch Iām not sureā¦I honestly donāt think would care, sheās very easygoing in a lot of ways. I just donāt know if she would be that comfortable in a slant since she is pretty long.
In looking at a lot of trailer designs, Iām thinking part of my issue is the ramp/curtain door configuration as well as the height of the butt bar. The ramp is steep and a bit short, so she ends up leaning right at the seam where the ramp meets the curtain doors, and is putting stress on the weakest part of the back. I may be better off with double doors and a ramp, or just a longer more shallow ramp that will reach higher up the back of the trailer.
Do you know someone with a slant or stock trailer you could try as an experiment? I have a 16.2 TB whoās pretty āaverageā backed and he rides in my 2 horse slant with lots of room. As in, he could be a full 18ā longer and not be squeezed in there.
I had a 17.3 behemoth before current horse who rode in same trailer. I locked the divider back for him and he just positioned himself however he wanted. He very rarely had to ride in one spot and it wasnāt like I had to take a running start to get door closed.
If you can, Iād try it just to see. Some horses really prefer to travel on a slant. Especially since your mare is sitting back that hard, Iād be super curious to see how she does in another set up.
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It was a straight load with a ramp. I always hauled her alone, she didnāt like companyā¦
Iām learning so much from this post. The following are just my thoughts.
I believe you mentioned youāve tried the four horse head to head facing back. If she is bracing on the back wall in that trailer I personally believe she will find a way to lean on the back/side of a slant. Iād think that would be the most dangerous position for both her and a trailer.
The very very best vets who I trust explicitly are only able to identify what they can see/feel. If itās a very subtle issue, maybe itās just not detectable.
My first thought is thatās just her thing. Especially since she doesnāt seem at all upset. If itās just her thing I immediately thought move the but bar forward.
Seems like a stock trailer tied like previous posters mentioned might be your best option. Donāt run out and buy one. Try to haul in one and see what happens. Now, legally, you canāt ride back there 
but seeing what sheās doing in a stock trailer would be ideal as long as you can be on the other side of divider for safety. Since you said she does this in a very short ride, maybe some big parking lot would be enough.
I thought that a stock trailer, by definition, DOESNāT HAVE a divider.
Stock trailers often have dividers that divide the front of the trailer from the back. Basically making a front stall and a back stall. Some have a divider that divides the very front (Which could store tack) from the open part that the animals are in. Depends on the length of the trailer.
Can you clarify this a little more? Do you mean mare will lean on divider or the actual wall of trailer thatās behind her? Because if itās the actual wall, sheād be able to do the same thing in a stock trailer if she got herself positioned right. Iām just confused why it would be dangerous.
I, of course donāt know what the horse will do 
In the slant trailers Iāve seen she would still be between dividers, just nose to one side of the trailer and butt to other at an angle. She could back up and wedge herself against the side of the trailer thatās in back of her and the divider. In the trailers Iāve seen the sides of the trailer are somewhat lighter duty construction that the back end of a two horse straight load. I donāt know if that would be more dangerous than any other way she might lean her bum, to me it just seems less stable.
I never thought of her leaning on the sides or back of a stock trailer. Of course she could do that too.
I have 2 mares that lean badly and brace against the butt bars. Once switched to a slant they stopped. One mare prefers to ride sideways, the other now prefers to face backwards, but sort of moves around. Neither lean against the trailer walls.
