The Romans, as they grew and conquered other lands, adopted those things from other cultures that they liked. They even adopted other peoples' gods. When the Romans met the Greeks and heard all the stories about the Greek gods, they adopted all the Greek gods. They changed the names of the gods to make them Roman and changed some of the stories to make the gods act more like Romans. Soon they even were able to act like these had been Roman gods all along.
If you want to learn more, read: The Truth About Myths