The Spirit Beast is an ancient demon that was sealed away long ago. It was attempted to be revived by Genshou Ryudoin.
Appearance
The Spirit Beast takes the form of a gigantic purple catfish with arms and legs. It has yellow eyes and long barbels. One of its most distinguishing features is the extra mouth it has located above its main mouth and below its eyes.
Part I
Naruto: Path of the Ninja 2
The Spirit Beast was sealed away deep in a cave located in the Valley of Evil. Genshou and his followers unsealed it, causing it to unleash a dark fog that poisoned the minds of those who entered it. Within 7 days, the fog would have spread enough to reach Konohagakure. Genshou planned to use the beast to destroy the world and start civilization anew with him as ruler. Naruto and his group of Sand and Leaf ninja attempted to seal it using the five spirit mirrors, but failed when Genshou, in a last ditch effort, fused himself with the beast. The group then battled the large beast, defeating it, presumably killing it once and for all.