Based on the information you've provided, my initial reaction is there might be some red flags, and I'd be worried about the owner having ulterior motives that may not be in your best interest. The owner may be looking to cut a deal now to gain more time to try to sell the horse for more money than the lease covers.

I'll preface everything with I'm unsure where you're located — but in my experience in the US, a year lease is about 1/3 the purchase price. I saw you mention the lease payment is $15k— if the horse were to sell tomorrow, is it actually worth $65k? Does it have a show record and experience to justify the fees?

• Does this lease require an upfront fee (kind of like a down payment) in addition to a monthly fee?
• Is the owner actively marketing the horse for sale— is the horse listed somewhere online for sale currently?
• By "show expenses covered" what does that include, exactly? I'd want an itemized list of everything covered by that statement included in the contract.
• Are you able to show enough to make "show expenses covered" worth it? If you're in Florida and have access to numerous shows through the year, then maybe it does make sense. But if you're somewhere in the midwest and it's winter, there's limited indoor show options (assuming you're in the US)
• Does the trainer / owner have a show schedule already planned? Is the facility you're going to a competitive show barn with the record to prove it? Can you attend upcoming planned shows if there's a schedule?
• What happens if the horse is injured during your lease? What options are outlined in the contract?
• Are you paying for board, training, vet, farrier, etc. in addition to the lease payment as well? Do you have to carry insurance for the horse?
• Have you asked yourself: is the horse really the best at it's job, the best you can get for the money, and the best fit for you and your goals?
• Do you like facility, the owner, the trainer, the people you're working with closely? Do you feel they have your best interest in mind, too? Because the lease also ties you to the facility in a way that may make it hard to leave (unless you're ok losing out the money) if things go badly.

Good luck! At the end of the day, my advice is get everything in writing and included in the signed contract.