I knew about the hippocampus in the brain, but until I started reading The Darwinian Tourist: Viewing the World Through Evolutionary Eyes, by Christopher Wills, I didn't know that seahorses are in the genus Hippocampus. One of the things I'm enjoying about Wills's excellent book is that he usually lists the scientific names of the living things that appear in the illustrations. The pygmy seahorse Hippocampus bargibanti, a tiny warty creature, shows up on page 17.1 (The warts help them blend in with the bulbous sea fan [similar to a coral] on which they live.)
The hippocampus in the brain is a structure that's involved in forming new memories and in spatial navigation. (Actually, it's a pair of structures, one on each side.) I knew the Greek root hippos (horse), but it had never before occurred to me to wonder if the horse had anything to do with the hippocampus. In fact, it does. In shape the hippocampus resembles the seahorse, and the scientific name Hippocampus is linked to the seahorse's resemblance to the terrestrial horse.
Other than its head, the seahorse doesn't resemble a horse at all. The source of the other half of the name Hippocampus, the Greek root kampos, is generally translated as sea monster. In Greek mythology, the Hippokampoi were large creatures with the front end of a horse and the back end of a fish; they provided transportation for the Nereids and pulled Poseidon's chariot. However, kampos may be related to kampe, or caterpillar, so you could also imagine seahorses as a horse/caterpillar cross, although that's not quite as appealing a notion.
You also see the root hippo in hippopotamus, or river horse. An 18th century anatomist, evidently in a state of confusion, referred to the hippocampus as the hippopotamus, and this confusion persisted for some time afterward. I don't know why the thought of a hippopotamus in the brain is funnier than the thought of a seahorse in the brain, but it is.
1 Unfortunately, Wills gives its name as Hippocampus bargobanti; let's hope that will be fixed in the Kindle edition and future paper editions.↩