The Whopper (「畳頭」?; Jōzu) was a giant fish dwelling in the Deep Abyss in Agata Forest in Ōkami. It was known to be extremely ravenous and was thus feared by the inhabitants of the forest. Whopper was renowned for swallowing the moon's reflection on the surface of the lake, capturing Yumigami, who had been residing in the image in the process.
Whopper was an enormous fish, large enough to eat a dog whole. Physically, it resembled a giant red catfish. The fish had extraordinary eating abilities, shown when he ate the moon's reflection, which is normally intangible, and even containing a god in its stomach.
Whopper had been residing in Agata Forest for an unknown length of time. At one point, the moon goddess Yumigami came to reside in the moon's reflection in the lake where Whopper was dwelling. In an exceptionally gluttonous moment, Whopper managed to consume the moon's reflection, trapping Yumigami in the process. Yumigami remained in Whopper's stomach, waiting for Amaterasu to free her.
Whopper continued to live in the pond for many years. However, it started to stir shortly after Amaterasu defeated the Spider Queen and cleared up Taka Pass. Eventually, the great fish burst forth and managed to consume Kokari's dog, Ume. Kokari attempted to capture the fish, but was unable to do so with his ordinary fishing pole and eventually ran out of line. Luckily, Amaterasu was passing by on her quest to find the Eight Canine Warriors. Together, Amaterasu and Kokari managed to capture Whopper. Dying, whopper released the moon it had eaten many years ago onto the surface of the lake, simultaneously releasing Yumigami, who taught Amaterasu Crescent. In its death throes, the fish spat out Ume, who turned out to be one of the Eight Canine Warriors.
Spoilers end here.
Whopper is never given a specific species in the game, although physically it resembles a catfish.