What do I do when they want their gift back?

I was given an animal three years ago. I have the bill of sale signed by original owner, but not signed by purchaser. I have original vet bills from original owner. I have paid all vet, food and upkeep for three yrs. Friendship with the "gifter" has ended. Recieved a certified letter that I am to return the animal, my free lease is up. There was never a lease agreement. I have a text from "gifter" stating they are not trying to keep me from my animal. I have notorized statements from friends and family that this "gifter" made statements to them that the animal was mine. Will I win in court?