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.

Proto-Indo-European root meaning "horse." Perhaps related to *ōku- "swift."

It forms all or part of: alfalfa; Eohippus; equestrian; equine; equus; hippo-; hippocampus; Hippocratic; Hippocrene; hippocrepian; hippodrome; hippogriff; Hippolytus; hippopotamus; Philip; philippic; Philippines; Xanthippe.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit açva-, Avestan aspa-, Greek hippos, Latin equus, Old Irish ech, Old Church Slavonic ehu-, Old English eoh, Gothic aihwa- all meaning "horse."