In some accounts I've read, the Greeks thought Apollo drove the chariot of the sun across the sky, in others it was Helios. Obviously they didn't see two suns rise and fall back then; How did these conflicting versions come about? Did one god originally get the credit and if so, when did lore of the other start to be told?
Apollo is initially the god of song, music and poetry, but considered as a solar-god too. Since it is one of the 12 leading god, he took the place of Helios many times.
This confusion comes from a nickname, Phoebus, which means "the shiny". The 2 gods had this nickname.
But the one who was driving the chariot of the sun, pulled by 4 horses (Pyroïs, Eoos Aethon and Phlégon), was Helios, which was going from the island of Aea to the land of the Hesperides.