My idea is a new mob I call an "Enderdog". It's pretty stupid, but I don't care.
Enderdogs are basically a re-skinned version of the dog, but in the end. They spawn in/near end cities along with enderman. If a nearby enderman attacks or gets attacked by something, they will teleport to the enemy and, like tamed dogs, kill them. If you kill them, they have a small chance of dropping an "Enderbone".
An enderbone is a darker and ender-ier bone that can be fed to your dog to give it a one-time special ability. If the enderbone-fed dog is sitting anywhere in the world (excluding other dimensions), and you get attacked, that dog will teleport to you, kill the enemy, and go back to where it was. A dog can only have 1 enderbone at a time.