Giant Multi-Headed Dog

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

Appearance
Its fur was all brown-coloured and it had a giant black receiver stuck through its head. It also had black colouring around its mouth along with razor sharp teeth and fangs.

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 gained additional heads every time it splits in this way, and each duplicated head carried the black receiver and the Rinnegan like the original. According to Nagato, the only way it can be defeated was by defeating its original summoner. In the anime, this summon was seen at different sizes, ranging from several stories tall to just tall enough to fit underneath trees.

This dog was also physically strong and resilient, since it broke through an ANBU Captain's Earth Release: Sticky Earth Drop technique, and another's 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 Technique. Even a usually devastating attack, such as the Wind Release: Rasenshuriken couldn't kill it easily, as the dog merely continued to multiply. Even when hit by a technique like Amaterasu, while it was incapacitated it didn't vanish until after Nagato himself was struck.

Tale of Jiraiya the Gallant
It was first summoned during Pain's fight with Jiraiya, but Gamaken was able to fend it off.

Six-Tails Unleashed
In the anime, it appeared before Tsurugi and his hunter-nin unit and attacked them.

Pain's Assault
It was summoned again during his attack on Konoha, where it fought against Gamaken, Gamabunta, and Gamahiro. They were unable to prevent it from splitting and it only disappeared once Naruto defeated the Animal Path.

Fourth Shinobi World War: Confrontation
It was summoned again during Nagato's and Itachi Uchiha's fight with Naruto Uzumaki and Killer B. Naruto attacked it only for Nagato to remind him that it can't be beaten directly. Later, after Itachi was freed from Kabuto's control with the use of a powerful genjutsu and his crow, he used Amaterasu on the dog, incapacitating it.

Trivia

 * This animal summoning had the qualities of two Greek Mythological creatures, the Lernaean Hydra (its abilities to produce more heads) and Cerberus (its appearance of a dog with several heads).