I'm not sure if you are asking for an actual family name (which I would have
to look up - Hippocampidae??) or whether you are just asking in general
terms where seahorses fit into the animal kingdom.  You'd be surprised how
many people don't know that.  In fact, some people think that seahorses are
invertebrates.  Actually, seahorses are bony fishes, right up there with
bass, cod and flounders.  Their closest relatives are the pipefishes, which
are also very interesting to observe.

I have seen seahorses on the south shore of Cape Cod on two occasions, both
times in association with floating mats of Sargassum weed which had drifted
north from the Caribbean in the Gulf Stream, then blown into shallow water.
We get quite a few tropical visitors showing up on the south shore of Cape
Cod during  the summer months, from that same source.  Portuguese man of war
are one such non-native species that sometimes appears.  Pipefishes are
quite common here in summer, but seahorses are only occasional visitors.

Regards,
Paul M.