Poseidon was the greek god of sea, after the Roman empire took everything in the region they also decided to change their gods with the dodekatheon (12 greek gods that did all the work). So the changed the name to something they would feel familiar with, as Neptune. They did so to other gods as well, such as Athena now called Minerva. As for why i did this post, i just had 3 minutes to spend for no reason.(better than working)