sea horse vs pipefish

sea horse and pipefish both are nouns.

Word	Noun	Adjective	Verb	Adverb
sea horse	Yes	No	No	No
pipefish	Yes	No	No	No

As nouns, pipefish is a hypernym of sea horse; that is, pipefish is a word with a broader meaning than sea horse:

sea horse: small fish with horse-like heads bent sharply downward and curled tails; swim in upright position
pipefish: fish with long tubular snout and slim body covered with bony plates

Other hypernyms of sea horse include needlefish.

sea horse (noun)	pipefish (noun)
small fish with horse-like heads bent sharply downward and curled tails; swim in upright position	fish with long tubular snout and slim body covered with bony plates
either of two large northern marine mammals having ivory tusks and tough hide over thick blubber