The story
From two-legged + eel, referencing this animal's eel-like shape and number of limbs.
Etymology from Wiktionary, CC BY-SA 4.0.
two-legged eel
Today it means: A species of aquatic salamander native to the eastern United States and northern Mexico, Siren intermedia, which has an elongated body possessing only two limbs: a pair of four-toed legs located behind the base of the head. Full definition →