Fastitocalon

A monstrous creature of the Sea said to be the last of the great 'turtle-fish'. Fastitocalon was so huge that sailors could mistake his immense shell for an island, but after they landed, the monster would dive into the depths of the ocean, drowning his unfortunate passengers. In Tolkien's world, the name Fastitocalon is known only from Hobbit folklore, and it's unclear whether it referred to an actual creature, or was merely a garbled misinterpretation of old sailors' tales.

Tolkien did not invent the word Fastitocalon; in fact it is found Old English poetry, where it seems to refer to a whale. Etymologically, though, the name ultimately derives from the Greek word for 'turtle', and Tolkien seems to have combined the two concepts, hence the notion of the 'turtle-fish'; a creature as large as a whale but with a horny shell on its back.