Hippocampus, which literally means “horselike sea monster,” was also another word for a seahorse. The part of the brain is named after its resemblance to a seahorse, though the term was debated for many years before it became more common in the medical field.
I know, this one seems sort of obvious—and maybe even common knowledge—given the compound structure and recognizable roots. But the context surrounding why the hippocampus got its name is surprisingly fascinating:
As you may know, the hippocampus is a component of the vertebrate brain located in the medial temporal lobe. As part of the limbic system, it plays a role in controlling emotion, memory and the nervous system. It is vital for consolidating information from short-term memory to long-term memory and for spatial navigation, so damage to it (such as that from Alzheimer’s or oxygen deprivation) results in memory loss and disorientation.
The word itself is an English adoption of the Late Latin hippocampus, from the Greek hippokampos, which is comprised of hippos (“horse,” as anyone with any familiarity with etymology probably already knows) + kampos (“sea monster”). (Interestingly, kampos is also likely related to the Greek kampe, or “caterpillar.”)
In Greek and Roman mythology, hippocampus/kampos often referred specifically to the aquatic equestrian creatures that pulled Poseidon/Neptune’s chariot. They are often depicted like so:
So what do horselike sea monsters have to do with brain bits?
Well, in Greek, Latin, and even historically in English, hippocampus meant “seahorse,” due to the fact that seahorses are tiny adorable horsey-looking sea monsters. Hippocampus is also the name of the genus to which all seahorses belong.
In 1587, Julius Caesar Arantius (a.k.a. Giulio Cesare Aranzi, 1530-1589), a surgeon and anatomist whose pioneering work in the field included discovering different anatomical structures of the human body, discovered the part of the brain in question and compared its appearance to—you guessed it—a hippocampus, or seahorse.
However, Arantius also debated whether it looked more like a seahorse or a silkworm—and the anatomists who continued his work through the early- to mid-1700s continued to debate whether the peculiarly-shaped protrusion of the hippocampus ought to be referred to as a silkworm, a dolphin, or a ram’s horn.
Indeed, the scientific term cornu Ammonis—meaning “Ammon’s horn” and harking back to the Egyptian god Ammon (or Amon), who often appeared in the form of a ram—is now used to refer to different parts or “fields” of the hippocampal formation.
Useless Etymology 