If there was an agreement that you would get the cat, and/or it was your personal cat, not a joint cat, then he way to get the cat from him would be to sue him you would sue based on breach of contract violation of the agreement or for a form of theft e.g. "conversion," or holding onto property which belongs to you but which he had custody of. A lawsuit is the only way to get back property when there is a dispute over who as the right to it.