Is a seahorse a plankton

A seahorse isn't a placental mammal; it's a type of fish, genus hippocampus. Seahorses are part of the same order as pipefish, Syngnathiformes. The seahorse's head is shaped like that of a horse.

Seahorse can, however, be another name for walrus, which is a placental mammal.

The hippopotamus was also called a seahorse during the 1800s.