Maybe there’s some confusion because they have “horse” in the name. Horses are mammals because mammals have mammaries. Where there’s milk, there’s mammals.
If you’re like me, you’ve always been a little more comfortable with mammals than all the other options for living creatures. Eggs and cell division and spores just seem a little off, a little like science fiction. It seems so much more correct to birth a live baby and feed it from your own food factory. I know, I know. I’m mammal-centric and I’m not sorry.
Anyhow, the seahorse is a weird little fish, but it is indeed a fish. It breathes water through gills. It has no mammaries and breeds with an egg laying ritual that’s a bit different, but still involves the cold randomness of eggs that may or may not spark a little waterlogged life.
Mammals can be sea creatures, of course. Whales and dolphins are both mammals who produce milk and breathe air instead of water.
My favorite theory about dolphins is that they were dogs who evolved into land creatures. The dogs were were drawn back to the water and adapted to swim better and better, eventually forming flippers and blowholes and cute clicking noises instead of barking. I’m sure I read that somewhere, unless I just made it up.
Look, I’m not a marine biologist, yet, but you can trust me on the seahorse thing. You can also trust me when I say that seahorses are carnivores. Yikes!
If you need more evidence, please don’t bother any actual seahorses, just read more from actual marine biologists, like these:
Feel free to add a question below in the comments!!