A hippocampus is a sea horse:
this one can be seen in the Roman museum at Bath, England, near the warm spring dedicated to the goddess Sul
this one's part of the Trevi Fountain in Rome, finished in the 1760s. Photo by Alexander Augst
Usually today we think of sea horses as rather different things:
photo: By © Hans Hillewaert, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=22106851 This is a short-snouted sea horse
but these sea horses are indeed still hippocampi because hippocampus the name of the sub-family of fish to which they belong.
We may not think about hippocampi very much, but hippocampi are always in our minds - well, in our brains, anyway. The hippocampi are two vaguely sea horse-shaped parts of the brain, one in each hemisphere of the brain:
illustration by Henry Vandyke Carter
They help with long and short-term memory, and with spatial memory, too.
I've long loved the sea-monster kind of hippocampi, especially since I've discovered that sea horses are too small for a human to ride, but what got me thinking about them recently was coming across the compound noun horse marine.
A horse marine is a mounted soldier who travels in a ship.
This is quite as odd as any of the other kinds of sea horse; but not nearly as romantic.
Word To Use Today: hippocampus. This word is Greek. Hippos means horse, and kampos means sea monster.