The giant oarfish (Regalecus) is a kind of seahorse!

The giant oarfish
(Regalecus glesne, Ascanius 1772; Figs. 1, 2) can be one of the longest bony fish in the sea, with the record holder measuring 11 meters. More commonly 3m is a typical length.

Until a closer sister taxon is added
to the large reptile tree (LRT, 1512 taxa) the giant oarfish nests with the seahorse (Hippocampus, Fig. 3). The oarfish can undulate its entire tail, like most fish. It can also undulate just the dorsal fin while the body itself remains essentially straight and motionless, much like a seahorse. The skin of is scaleless, but covered with tubercles (like sticklebacks and seahorses). The oarfish has been seen swimming vertically, tail down, like a seahorse. The short pectoral fins are vestiges. The long pelvic fins with oar-like tips are not used for locomotion, but trail behind.

Traditional cladograms
nest seahorses within the clade Percomorpha (perches, pufferfish, anglerfish, tunas, cichlids, etc.) distinct from the the clade Lampriformes (opahs, oarfish, etc.).

At present
the clade Percomorpha (Perca at its base) is confirmed, sans pufferfish (Diodon), within the LRT (subset Fig. 4).

The clade Lampriformes is not confirmed as monophyletic.
Opah nests with flounders. Oarfish nest with seahorses.

The skull is strange
and hard to score (Fig. 2). It is best understood like a seahorse with a short, bulldog-like rostrum, distinct from most long rostrum pipefish and sea horses.

Let’s start with the fact
that the mouth is vertical and the premaxilla is dorsal, distinct from most vertebrates, except seahorses. The pectoral girdle is fused to the pelvic girdle. So the giant oarfish is nearly all tail.

The eggs are 2.5mm in diameter.
Diet includes krill, small fish and squid, when large enough to prey on those items.

Other common names for the giant oarfish include:
Pacific oarfish, king of herrings, ribbonfish, and streamer fish.

References
Ascanius P 1772. Philine quadripartita, et förut obekant sjö-kräk, aftecknadt och beskrifvet. Kongliga Vetenskaps Academiens Handlingar 33 (10-12): 329-331, pl. 10.,

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