I have a horse I am training and planning to sell in the future. Can I write off training fees?

Yes, if it is an ordinary and necessary expense related to a business.  If training fees are an ordinary expense for your horse business, then yes you can deduct them on the Schedule C.  You will want to make sure this is not a hobby of yours and is a bona fide business.  Hobby losses are not deductible.  The other option would be to capitalize the costs by adding them to the cost you paid for the horse which would decrease any gain or increase the loss on the sale of the horse.  Click here for more Publication 535 that discusses horse expenses.