I realized I didn't know much about seahorses, so I decided to look them up. Turns out they are fishes. They're a heavily modified kind of pipefish, which are a genus of fish that ... well, they're basically straightened seahorses. They're called pipefish because they have tiny little mouths.
I was a little confused because seahorses really don't look like fish. In fact, they look kind of like tiny little horses, only in the sea. Hence the name. Seahorses. They're bony fish, but they don't have scales. Also, they swim upright, unlike most fish. Also, they have flexible necks, unlike most fish. Frankly, the seahorse is one twisted little dinkus. They suck at swimming, so mostly they sit in one place with their tails wrapped around something to hold them there. They're tiny, carnivorous, ambush horses. Of the sea. They eat shrimps and stuff.
Oh, and the males get pregnant, which is pretty cool. And their genus name is hippocampus. From the Greek. For Horse + Seamonster. They're horsey seamonsters. Literally.
Here's a picture I drew of a seahorse.