Why is seahorse a misnomer?

Technically it isnt. Seahorse is the name given to 54 species of small marine fishes in the genus Hippocampus.

"Hippocampus" comes from the Ancient Greek Word hippos meaning "horse" and kampos meaning "sea monster" therfore horse sea monstier. The Word "seahorse" can also be written as two separate Words (sea horse), or hyphenated (sea-horse). Its names is derived following discovery because of the a head and neck suggestive of a horse