Why is the ninth month called September?
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.
hippocampus hip·po·cam·pus (hĭp'ə-kām'pəs)
n. pl hip·po·cam·pi (-pī')
The complex, internally convoluted structure that forms the medial margin of the cortical mantle of the cerebral hemisphere, borders the choroid fissure of the lateral ventricle, is composed of two gyri with their white matter, and forms part of the limbic system.
Plural hippocampi (hĭp'ə-kām'pī')
A convoluted, seahorse-shaped structure in the cerebral cortex of the temporal lobe of the brain, composed of two gyri with white matter above gray matter. It forms part of the limbic system and is involved in the processing of emotions and memory.