Jim was just a dirt-eating, chewy length of worm flesh, but all that came to a crashing end...
Abyss Worm -- Bloodworm -- Flood Worm -- Gigas Worm -- Lava Worm -- Purple Worm -- Roundworm -- Sandworm
Most likely inspired by Dungeons and Dragons' purple worm and giant bloodworm, which are both basically giant, killer worms. Bloodworms are real animals and get their name from their red colour, although the giant bloodworm is described as "mottled green" and is named for its ability to drain blood from its victims.
Another giant worm is the minhocao of South American folklore, reports of which date from the late 19th century.
IMAGES: A purple worm from D&D -- Bloodworms
Roundworms are parasitic worms, so called because of their unsegmented, cylindrical bodies, that live in the intestines. The name fits, since you fight the roundworm inside Leviathan.
IMAGE: Roundworms
Sandworms are real animals, although this monster could also be based on the sandworm from Frank Herbert's Dune novels. Having never read the books, I can't say how similar the two sandworms are. Another possible inspiration is the thunderherder, a desert-based version of the purple worm from D&D.
IMAGES: A sandworm from the 1984 film adaptation of Dune -- Real sandworms
I can't find any direct basis for these creatures, but there are earlier references to giant worms mentioned in the entries for the purple worm and sandworm. Gigas is Latin for giant.
IMAGES: A purple worm from D&D -- A sandworm from the 1984 film adaptation of Dune
Back to...PURPLE WORM, BLOODWORM

ROUNDWORM

SANDWORM


ABYSS WORM, FLOOD WORM, GIGAS WORM, LAVA WORM

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