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