Stand on his doorstep and scream til he brings your dog out. If you have proof of ownership can you not call the police?

Hand over the dog and no one gets hurt?

I'm hoping you have taken that dog to a vet to get its shots. That would also prove it's yours. The ex sounds very emotionally immature. Good riddance.

So your ex is an asshole

Another person complaining about the fallout of dating some douche, ydi.

OP, if this dog is a purebred from a breeder, I would think you have it registered with a kennel club. My guys are both registered under my name with the AKC (American Kennel Club) and the UKC (United Kennel Club), the two biggest KCs in the US so we can compete at events for both. Your breeder will also have the ownership paperwork, as should you, from when you bought the dog. If it's a shelter/rescue dog, you should also have that information from when you adopted the dog. It would also have been a good idea to register a mutt with the AKC under your name as mutts can compete in everything but conformation. Did you have the dog microchipped or tattooed? Whose information is attached to these? Do you have paperwork from the vet from his annual vet visit or his required rabies vaccine visits? I don't remember how often you have to go in for NY. Where I live it's every three years, but in other places, like Florida, it's annually, so you should have that paperwork somewhere if you have had the dog for three years. I hope something in here has given you an idea of where to find proof that the dog is yours that you can take to the authorities if he refuses to give the dog back. For future reference, I keep all of this info together in a file for each of my animals (dogs and horse). I have multiple files with this information as well as current photos of the animal so that I can whip it out and show a nice, clear photo of the animal from different angles. I have photos of any unique markings on each animal. I originally only did it for my horse as horse thieves are, sadly, still a thing, but it's a good idea to do it for your others as well. This is why I am all for dogs being considered property. We have laws that protect them from abuse (when enforced) unlike we do with inanimate objects, yet they are still property with paperwork and proof of ownership which means that no one can try to claim my dog. We will all be in trouble when dogs are no longer considered property. Mind you, my dogs are spoiled rotten and I don't treat them like "property", but I own them and no one can dispute that. OP, I hope you are able to get your dog back from this asshole.

All these comments on how to get your dog back and I'm sitting here wondering why you didn't take it with you when you left in the first place

i don't think he broke up with you because of the dog

Call the cops. If you have any of your vet papers or adoption papers stating you as the owner, then he has to give the dog back or the cops can arrest him for theft.

This can be taken to court if it can't be solved on your own.