Monotheism seems to take root in the Upanishads, which seems to predate any other written belief system. There is some confusion because the Vedas and Avestas are basically saying the same thing, so the source is debatable, ultimately. What has happened is that we have centered on the appearances of the One, rather than the One itself. To be clear, though, Hindus don't have many Gods, the Hindu has one God taking many forms. There are so many because whatsoever you think of as God is not God, the thinker of that is. The appearance of your ideal has no bearing on the Absolute. It is only in the transcendence of partiality that God is actually known.