Dread Turtles, a race of fearsome reptiles with spiked shells, are usually found near water. Though feared by many, they are prized by certain hunters for their meat and their impenetrable shells which can be made into shields. They are in the area around Nen Harn in Bree-land, on Tyl Ruinen in Evendim, the Dunbog in Dunland, and are Elite-level creatures in Malenhad in Angmar. The creatures of Dragon-kind known as Avancs have spiked shells which suggest a possible kinship with Dread Turtles.
Hobbits have a popular poem called "Fastitocalon" which tells of an enormous "turtle-fish" mistaken for an island by unwary seafarers. This presumably was inspired by the Dread Turtles. In fact, there is a Dread Turtle in West Rohan of incredible size that is referred to as the Fastitocalon, though none can say if it is the same beast of the ancient legend.
Dread Turtles are "normal" or Elite and tend to threaten before aggroing. They are resistant to Cries and Common damage.
The Hobbit poem "Fastitocalon" appears in The Adventures of Tom Bombadil. However, J. R. R. Tolkien does not elaborate on whether such creatures truly existed in Middle-earth or if they were simply legendary.