Turtle-fish were a legendary race of giant sea monsters, mentioned only in The Hobbit verse. The last of the Turtle-fish was named Fastitocalon.
Turtle-fish appeared as treacherous islands, capable of drowning seafarers.
Quinton Hoover - Fell turtle
Inspiration
J.R.R. Tolkien was inspired by an Old English Beastry, appearing in the Exeter Book. The idea of giant turtles ultimately derives from Eastern myths, and in the European versions the monster is often confused with whales.
Other version of the legendarium
It is never explained whether the turtle-fish was an actual race or just a fictional character for the poem "Fastitocalon". In an earlier version of the Legendarium, Ulmo, Lord of the Waters, uses his giant whale, Uin, to drag Tol Eressëa towards Aman. This whale reappears in J.R.R. Tolkien's story Roverandom. It is thinkable that Tolkien, in an early phase of the mythology, imagined the fastitocalon to be another one of Ulmo's creatures.
Portrayal in adaptations
Turtle-fish, also called Fell Turtles and Festitycelyn, are described as monsters that can grow almost as large as fifty feet in length, and that live both in the seas and in the large rivers and lakes.
References
J.R.R. Tolkien, The Adventures of Tom Bombadil, "Fastitocalon."