You might be able to swing it
Only because there are no more intelligent replies, I'll mention that a free lease could be compared to a deed of real property in which the transferor/grantor retains a reversion upon occurrence of certain event (time lapse, failure to abide by contract, etc.) and these do occasionally qualify for reduced taxation. It will depend upon your state, the terms of the lease (it should definitely be in writing, and proof of the lessee's charitable/ public policy use may be req'd), whether you itemize, IRS rules re: horses as property, your annual income, etc. So yes, tax preparer should know or find out - just mention to him it could be akin to a conditional alienation of the property/horse. If the lessee is an established non-profit or therapy center they may be able to help, too. It may not be a significant amount, but, again, this depends upon your particular tax circumstances.
I hope that made some sense, but doubt it did.
[this post is accompanied by the usual atty boilerplate - no warranties, I don't represent you, you should seek more competent counsel, etc.]
Best of Luck!