Oannes...Myth? Legend? Or did he really exist?

Illustration on page 495 of Curious Myths of the Middle Ages (Photo credit: Wikipedia)  

Who was Oannes?  Beings like Oannes remain mysteries from ancient history that have yet to be solved.  Did this mysterious creature really exist?

Oannes by Bruce Magnotti

Oannes as world traveler The most likely scenario, if Oannes was indeed a historical figure, is that Oannes was a traveler from another part of the globe, from a pre-existing civilization yet to be discovered. “A man, or rather a monster, Half man and half fish, coming from the sea, appeared near Babylon; he had two heads; one, which was the highest, resembled that of man, the other that of a fish. He had the feet of a man, and the tail of a fish; and his speech and voice resembled that of a man: a representation of him is still preserved. This monster dwelt by day with men, but took no food; he gave them knowledge of letters, arts, and sciences; he taught them to build towers and temples; and to establish laws; he instructed them in the principles of geometry; taught them to sow, and to gather the fruits of the earth; in short, whatever could contribute to polish and civilize their manners. At sun set he retired to the sea, in which he passed the night. There appeared likewise others of the same species.”
via: www.oannes.com