How did Capricorn get its name?
Capricorn is a wee bit confusing. Capricorn - Capricornus is a Latin compound of "she-goat" (capra) and "horn" (cornus), which translates the original Greek Aegocerus, "goat-horned." It is associated with being a Goat Horn, however it is also traditionally depicted as being a "sea goat" (a goat with a fish tail).
According to Theony Condos (Star Myths) the only ancient allusions to Capricorn are in relation to astronomical explanations (i.e. no regular Greek myths explaining why this constellation is in the sky.). Astronomical sources place this as being Aegocerus (goat - horned) who accompanied Zeus when battling the Titans on Mount Ada. Aegocerus allegedly invented the trumpet and then blew his trumpet in battle which caused panic among the Titans and so they fled.
There is what's thought to be a meager attempt in this version of the story to reconcile the "sea-goat" by saying that Aegocerus discovered the trumpet in the sea. The depiction of a sea-goat appears to be a hold over from an earlier Egyptian source.
According to Michael Macrone (By Jove) some later sources may have associated Capricorn with a goat that fed the infant Zeus as he was being secretly raised on Crete, the goat whose horn became the original "cornucopia" (horn of plenty).