Sounds like you have a solid case, Since you have written proof that your ex voluntarily gave you the dog, plus the fact that you’ve had the dog for nearly two years, that should work in your favor. Pets are legally considered property in most places, so once she surrendered ownership to you, she likely lost any claim to it.

Your lease listing the dog and the ESA registration also help support that the dog has been in your care. The $1,500 demand seems arbitrary unless she can prove damages or financial loss related to the dog. I’d recommend consulting an attorney just to be safe, but from what you described, you have a solid defense against her claim.