Giant Multi-Headed Dog

A large, multi-headed dog, this summon has a number of body piercings and Rinnegan eyes, implying it is being controlled by Pain. Using Amplification Summoning Technique, it can split apart, when struck, into individual dogs that can then merge back together. It gains additional heads every time it splits in this way. It is first summoned during Pain's fight with Jiraiya, but Gamaken is able to fend it off. It is summoned again during his attack on Konoha, where it fights against Gamaken, Gamabunta, and Gamahiro. They are unable to prevent it from splitting and it only disappears once Naruto defeated its summoner.

Trivia

 * This animal summoning has the qualities of two Greek Mythological creatures, the Hydra (it's abilities to produce more heads) and the Cerberus (it's appearance of a dog with several heads).
 * This dog may be a ninken.