Giant Multi-Headed Dog

A large, multi-headed dog with wings, this summon has a number of body piercings and Rinnegan, implying it is being controlled by Pain.

Abilities
It was modified by Pain using the Amplification Summoning Technique, so that 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, and each duplicated head carries the Chakra Receiver and the Rinnegan like the original. In the anime, this summon is seen at different sizes, ranging from several stories tall to just tall enough to fit underneath trees.

This dog is also physically strong and resilient, since it broke through the ANBU Captains' Earth Release: Sticky Earth Drop, Lightning Release: Sixteen Pillar Bind, and received no more than minor burns from Fire Release: Fired Pottery Technique despite having its legs cut off by the Wind Release: Wind Cutter.

Itachi Pursuit Arc
It is first summoned during Pain's fight with Jiraiya, but Gamaken is able to fend it off.

Tsuchigumo Kinjutsu Arc
In the anime during the Tsuchigumo Kinjutsu Arc, it appears in front of Tsurugi and his hunter-nin unit and attacks and kills them.

Invasion of Pain Arc
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 (its abilities to produce more heads) and the Cerberus (its appearance of a dog with several heads).