I'm going to go with almost def a bullfrog. Looks just like the bullfrog tadpoles I raise every year. That one looks like it's about to go frog on you soon. Might be a good idea to put a large piece of wood or a rock that peaks out of the water in the tank. I find mine like to rest on it when they first transform. I think it must be a tiresome process for them. You will want to make sure you have a good solid lid on the tank though...those guys have some powerful legs and giant sprinting hops. If you want to keep it as a frog you will need a big tank too. They are not tiny frogs.