My ex and I got a puppy, four years later the relationship ended. We were passing the dog back and forth for a while but now that I am in a new relationship she all of a sudden do not want me to get her anymore. What do I do?

You go to Court - probably Small Claims - and ask for the return of the dog. Animals are basically property, the same as a chair, so the Court will evaluate the circumstances, the expenses, the care given to the animal, and then award the dog to one of you.