Like many cultural items, the Romans borrowed the Greek Poseidon for their god Neptune. In earlier forms, Poseidon dates back as far as Greek culture can be traced (c. 1450 B.C.), but his story as we know it today comes from Homer around 700 B.C., where after overthrowing their father Cronus, Zeus, Poseidon, and Hades drew lots for ruling the world. Zeus got the sky, Poseidon the seas, and Hades the underworld.
The actual images of the gods are just idealized humans, in the way Hemsworth and Evans are in the MCU, or body-builders and football players are in mundane sports. Classical mythology assumed the gods had "perfect" and immortal bodies, but they shared the dysfunctional personalities of mortals. That made for better story-telling. If the artists and sculptors used people as models, they just used good-looking ones and then enhanced them to look heroic.