The story
From Spanish (adjective) + dancer (noun), probably from the resemblance of the animal to a flamenco dancer in a reddish dress with ruffles.
Etymology from Wiktionary, CC BY-SA 4.0.
Spanish dancer
Span·ish danc·erToday it means: A species of nudibranch sea slug, Hexabranchus sanguineus, commonly growing up to a length of 25 centimetres (about 9.8 inches), with some in the Red Sea up to 40 centimetres (15.7 inches). Full definition →