What should I do about this pony leaving?

Definitely talk to the owner of the Pony. Whether they are boarding the pony there or if it is the barn's pony, you need to tell them what you think. If anything they can delay him going away until a decision can be made. You can make medical decisions, and if you take the best care of him you shouldn't have to make many. But it'll be your "child" so tell your parents that it'll be your decision to make with the vet.