Your horses are not inventory, they are tangible assets as I understand it.
Unless the horse is over 12 years old when acquired, it is depreciated using a seven year schedule. Use the 150% declining cost recovery system (MACRS), If over 12 years old then use three year depreciation schedule.
This is the best I could find about horse taxation
https://lawtaxprof.com/?cat=3#:~:text=The%20asset%27s%20useful%20life%2C%20where,depreciated%20as%20....
I would print that web page to pdf if it were me, pages tend to get deleted sometimes so I would want it on hand for future reference
Donations are always at either a specified cost the donator provides, or the fair market value at the time of the donation.
The IRS has a de minimus limit of 2,500. If the horse costs less than that amount then yes you can expense it. But if you do then there is no annual depreciation on the horse.
