oarfish

(ôrfis̸h′)

noun pl. oarfish, oarfishes

any of a family (Regalecidae, order Lampriformes) of large, long, narrow, deep-sea bony fishes having a fin along the length of the back and a manelike crest behind the head: some reach a length of 9m (c. 29.5 ft) and are responsible for many reports of sea serpents

See oarfish in American Heritage Dictionary 4

noun pl. oarfish oarfish or oar·fish·es
A widely distributed marine fish (Regalecus glesne) having a slender silvery body up to 11 meters (36 feet) in length, a bright red dorsal fin along its entire length, and an undulating motion in swimming resembling that of a snake.
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