The word hippocampus was borrowed in the seventeenth century from Late Latin, when it referred to a type of mythological creature that was part horse and part dolphin, best known for pulling Poseidon's/Neptune's chariot. This was similar to the capricorn and also appeared in Etruscan and Pictish folklore. Later on, this term got adopted for the part of your brain that deals with memory and learning due to a perceived visual resemblance between the creature and the cerebral structure. The Latin word comes from Greek hippokampos, which is from the words hippos, meaning "horse" and kampos, meaning "monster". Hippos, also the root in hippodrome and the name Phillip, comes from Proto-Indo-European hekus, meaning "swift", and kampos might come from a word that meant "caterpillar".
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