hippocampus (n.)

c. 1600, a kind of sea monster, part horse and part dolphin or fish, often pictured pulling Neptune's chariot, from Late Latin hippocampus, from Greek hippokampos, from hippos "horse" (from PIE root *ekwo- "horse") + kampos "a sea monster," which is perhaps related to kampe "caterpillar." Used from 1570s as a name of a type of fish (the seahorse); of a part of the brain from 1706, on supposed resemblance to the fish.