Fastitocalon (poem)
From Tolkien Gateway
"Fastitocalon" is a poem by J.R.R. Tolkien about a beast of the same name. The setting is Middle-earth.
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.
It is never explained whether the turtle-fish were an actual race or fictional characters for the poem.