[2] All seahorse and pygmy pipehorse species have a prehensile tail (a character shared with some other syngnathids),[4] a fully enclosed brood pouch, a short head and snout angled ventrally from the abdominal axis, and no caudal fin.
[7] The subfamily Hippocampinae is named after the seahorse genus Hippocampus, which is derived from the Ancient Greek ἱππόκαμπος (hippokampos), a compound of ἵππος, "horse" and κάμπος, "sea monster".
Due to the morphologically intermediate nature of the pygmy pipehorses between pipefishes and seahorses, the taxonomic placement of this group remains contentious, and three different classifications have been proposed for the subfamily Hippocampinae.
[9] An alternative explanation for the evolution from pygmy pipehorse to seahorse is based on the finding that a vertically bent head is more efficient in capturing prey because it increases the animal's strike distance, which is considered particularly useful in tail-attached sit-and-wait predators.
[3] Independent geological confirmation of the genetic data would require finding a fossil site from the Oligocene in which seahorse-like pygmy pipehorses are present, but seahorses are not.