Giant Multi-Headed Dog

One of Pain's many summonings used to battle his former teacher, Jiraiya. It had the same Rinnegan-like eyes like Pain implying his control over it. Also it has the same studs as various Pain bodies does, that recieves some sort of chakra. Pain's giant dog has the ability to split into more dogs each time it attacked. These dogs can then merge and split apart at will. The more the dogs grew, split, and merged, the stronger it would get. It was used to battle Jiraiya's giant toad summon, Gamaken. It was strong enough to send Gamaken flying.

Invasion of Pain Arc
After Naruto's return to Konoha, Pain's new Animal Realm body summoned the dog once again to capture Naruto along with a Rhino and Yak. Naruto sends the dog to the sky with a Sage Giant Rasengan and has Gamabunta kill it. It splits apart when under Gamabunta's feet, resulting in both Gamahiro and Gamaken having to face them. However, before the dogs can attack, Animal Realm is killed and as a result the dogs are dispelled.

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).