As ROG says not illegal but not advisable.
Theory as to why here:
Think of the towball as a pivot point ...
If your horse moves backwards in the trailer the weight transfers to the back of the trailer in effect lifting the rear of the towing vehicle (not literally off the ground) but affecting the stability.
If your horse moves forwards this transfers the weight on to the rear of the towing vehicle which will have the effect of lightening the steering.