I don't want to give back their dog
Why would you want to keep someone's dog?
It's mine now, I love him and I found him.
That is their dog. You shouldn't keep something that doesn't belong to you.
Maybe I'll return the dog and maybe I'll keep him for good. Idk about that yet.
You'd better return it or you'll get into a ton of trouble. Why not just go and find your own dog?