Probably not.
It sounds like the horse is your personal animal and you use him for some 'extra money'. If that is the case, it is non-deductible.
If the primary purpose of the horse was for business, such as aggressively advertising year-around contracts (weather permitting) so that it becomes a primary source of your income, then maybe. However, if you need to keep him 'in shape' that seems like it may not be the case.
Horses (and airplanes) are common personal items the people try to claim are primarily for business, and they are very often audited (and lose).