The second-century Latin Physiologus tells of a sea-monster, the Aspidochelone. This is retold in the Old English poem "The Whale", where the monster appears under the name Fastitocalon, in the Exeter Book, folio 96b-97b.
By etymology, the name "Fastitocalon" is a corruption of the Greek Aspido-chelōne, "round-shielded turtle", with the addition of the letter F, according to Tolkien, "simply to make the name alliterate, as was compulsory for poets in his day, with the other words in his line. Shocking, or charming freedom, according to taste". Tolkien commented that the tale of the monster that treacherously simulates an island is from "the East", and that the turtle is mixed up with a whale when the story arrives in Europe, so that the Old English version has him feeding like a whale "trawling with an open mouth".
Look, there is Fastitocalon!
An island good to land upon,
Although 'tis rather bare.
Come, leave the sea! And let us run,
Or dance, or lie down in the sun!
See, gulls are sitting there!
Gulls do not sink.
There they may sit, or strut and prink:
Their part is to tip the wink,
If anyone should dare
Upon that isle to settle,
Or only for a while to get
Relief from sickness or the wet,
Or maybe boil a kettle.
Fastitocalon, the central character in the poem, is the last of the mighty turtle-fish. This poem is well known to the Hobbits. It tells of how Fastitocalon's huge size, a "whale-island", enticed sailors to land on its back. After the sailors lit a fire upon Fastitocalon, it dived underwater, causing the sailors to drown.
Fastitocalon was at the surface for long enough for vegetation to grow on its back, adding to the illusion that it was an actual island. Fastitocalon was far larger than the largest non-fictional turtle (Archelon).
It is never explained whether the turtle-fish were an actual race in Middle-earth or fictional characters created solely for the poem. It is distinctly possible that the story is in fact an allegory of the fall of Númenor. Like the Fastitocalon, Númenor too sank below the waves, and drowned most of its inhabitants.
Norma Roche writes in Mythlore that Tolkien makes use of the medieval story of the voyages of Saint Brendan and the Irish Immram tradition, where a hero sails to the Otherworld, for his vision of the Blessed Realm and seas to the west of Middle-earth, as seen in poems such as "The Sea-Bell" and "Imram", while (as several scholars note) his "Fastitocalon" resembles the tale of Jasconius the whale.
John D. Rateliff notes that Tolkien stated that when he read a medieval work, he wanted to write a modern one in the same tradition. He constantly created these, whether pastiches and parodies like "Fastitocalon"; adaptations in medieval metres, like "The Lay of Aotrou and Itroun" or "asterisk texts" like his "The Man in the Moon Stayed Up Too Late" (from "Hey Diddle Diddle"); and finally "new wine in old bottles" such as "The Nameless Land" and Aelfwine's Annals. The works are extremely varied, but all are "suffused with medieval borrowings", making them, writes Rateliff, "most readers' portal into medieval literature". Not all found use in Middle-earth, but they all helped Tolkien develop a medieval-style craft that enabled him to create the attractively authentic Middle-earth legendarium.
Paul H. Kocher comments that from a land-loving Hobbit point of view, the story warns never to go out on the dangerous sea, let alone try to land on an uncharted island. He groups the poem with "Oliphaunt", which the Hobbit Sam Gamgee recites in Ithilien, and "Cat", where the innocent-looking pet dreams of slaughter and violence, as reworked Bestiary poems.
- Albert Stanburrough Cook, ed. (1821). The Old English Physiologus. Volume 63 of Yale studies in English. Verse translated by James Hall Pitman. Yale University Press. p. 23.
- J. R. R. Tolkien, The Letters of J.R.R. Tolkien, #255 to Mrs Eileen Elgar, 5 March 1964; Humphrey Carpenter (1981) George Allen & Unwin; ISBN 0-261-10265-6)
- Christina Scull & Wayne G. Hammond (2014), editors, The Adventures of Tom Bombadil, Harper Collins, p. 224; ISBN 978-0007557271
- J. R. R. Tolkien, ed. Humphrey Carpenter (1981), The Letters of J. R. R. Tolkien, George Allen & Unwin, letter 255 (5 March 1964), p.343; ISBN 0-04-826005-3
- Adam Roberts (1 November 2013). The Riddles of The Hobbit. Palgrave Macmillan. pp. 78–38. ISBN 978-1-137-37364-9.
- Shippey, Tom (2005) . "Appendix B "Four 'Asterisk' Poems"". The Road to Middle-Earth (Third ed.). HarperCollins. p. 326. ISBN 978-0261102750.
- Roche, Norma (1991). "Sailing West: Tolkien, the Saint Brendan Story, and the Idea of Paradise in the West". Mythlore. 17 (4): 16–20, 62.
- Swank, Kris (2015). "The Irish Otherworld Voyage of Roverandom". Tolkien Studies. 12 (1): 31–57. doi:10.1353/tks.2015.0011. ISSN 1547-3163.
- Rateliff, John D. (2014). "Inside Literature: Tolkien's Explorations of Medieval Genres". In Houghton, John Wm.; Croft, Janet Brennan; Martsch, Nancy (eds.). Tolkien in the New Century: Essays in Honor of Tom Shippey. McFarland. pp. 133–152. ISBN 978-0-7864-7438-7.
- Kocher, Paul (1974) . Master of Middle-Earth: The Achievement of J.R.R. Tolkien. Penguin Books. pp. 189–190. ISBN 978-0-14-003877-4.