Spirit Beast

The Spirit Beast was an ancient demon that was sealed away long ago. Genshō Ryūdōin attempted to revive this beast.

Appearance
It took the form of a gigantic purple catfish with arms and legs. It had yellow eyes and long barbels. One of its most distinguishing features was the extra mouth, located above its main mouth and below its eyes.

Plot Overview
The Spirit Beast had been sealed away deep in a cave located in the Valley of Evil. Genshō and his followers unsealed it, causing it to unleash a dark fog that poisoned the minds of those who entered it. Within seven days, the fog would have spread far enough to reach Konohagakure. Genshō planned to use the beast to destroy the world and start civilisation anew with him as the ruler. Naruto and his group of Suna and Konoha ninja attempted to seal it using the five spirit mirrors, but failed when Genshō, in a last ditch effort, fused himself with the beast. The group battled the large beast, defeating it, presumably killing it once and for all.

Trivia

 * The Spirit Beast, like the Master Puppet, Three-Heads, and the Zero-Tails, has no specific name. It is also considered to be a massive super weapon capable of mass destruction.