Giant Squid

The Giant Squid is a creature that lives in the Hogwarts Lake. Despite the generalized fear of it, the Giant Squid is actually very docile and helpful to the students of Hogwarts, such as when Dennis Creevey fell into the Lake, and he was immediately picked up by one of the squid's tentacles and put safely back in the boat. It was also mentioned that the Squid played with the Students when they came to the Lake's shore.

Behind the scenes
J.K. Rowling revealed, whilst making a joke, that the Squid is none other than Godric Gryffindor, one of the four founders of Hogwarts School. This makes it the world's largest Animagus. On the eleventh hour of every evening, The Squid rises from the lake and transforms into Gryffindor, who wanders the school grounds before returning at sunrise.