Fastitocalon is the last of the mighty turtle-fish who is the central character in this poem of the same name. This poem is well-known to the Hobbits and tells of how because of his huge size, sailors landed on his back only to drown when he dove underwater.
Fastitocalon was the size of a small island and vegetation would often grow on his back when not submerged adding to his deceitful appearance as an actual island.
The fastitocalon reminds of the biblical/kanaanite leviathan.
It is never explained whether the turtle-fish were an actual race or fictional characters for the poem. In the History of Middle-earth 1: The Book of Lost Tales 1 however, Ulmo, Lord of the Waters, uses his giant whale, Uin, to drag Tol Eressea towards Aman. This whale reappears in Roverandom. It is thinkable in an early phase of the mythology Tolkien imagined the fastitocalon to be another one of Ulmo's creatures, still alive in biblical times.