If you acquired the horse specifically to meet the course requirement (you didn't already own it), I'm of the opinion that the entire expense would qualify for a 529 plan distribution.
I base that opinion on the wording in my previous reply:
The key word being "required". Qualified expenses for a 529 distribution are more liberal than for an education credit.
From Pub 970: "Qualified Higher Education Expenses are expenses related to enrollment or attendance
at an eligible postsecondary school"...."The expenses for fees, books, supplies, and equipment required for the designated beneficiary’s participation in an apprenticeship program".
For computers and peripheral equipment, it says "if it's to be used primarily by the beneficiary
during any of the years the beneficiary is enrolled at an eligible postsecondary school".
