sea mousesea mouse
any of a genus (Aphrodite) of large, marine polychaetes with a flat, oval body covered with bristles
Origin of sea mousefrom its bristles and general shape
Any of various large marine polychaete worms of the genus Aphrodite, especially A. aculeata, having a flattened elliptic body with overlapping scales covered by long hairs.
(plural sea mice)
Perhaps because it resembles a bedraggled mouse when washed up on shore.