Bull

is a ninken, and is a personal summon of Kakashi Hatake.

Personality
Befitting of his name, he is a large, intidimating black bulldog wearing a spiked collar. Unlike all the other members of Kakashi's nin-dog pack, Bull does not appear to have the ability to speak. Nonetheless, he and Pakkun seem to get along well enough for him to allow the latter to ride atop his head. Comparatively, Bull is the largest of Kakashi's ninken.

Land of Waves Arc
Kakashi called on Bull and the nin-dog pack to track and capture the Missing-nin Zabuza Momochi. They managed to bite onto the missing-nin and pin him in place. This allowed Kakashi to use Lightning Cutter without fear of a counter attack.

Sasuke Retrieval Arc
A short time later, Bull and his fellow nin-dogs would be called upon by Kakashi to track down Naruto Uzumaki and his team who had gone off in pursuit of Sasuke Uchiha.

Ultimate Weapon Arc
He along with the other ninkens were summoned by Kakashi to investigate the Takumi Village.

Rescue Gaara Arc
Several years later, Bull and the nin-dog pack were called upon to track down the Akatsuki member Sasori using a small piece of his clothing.

Hunt for Itachi Arc
Some weeks later, Kakashi summoned the pack to help track down Itachi Uchiha. Bull was partnered with Naruto, Hinata, and Yamato.