Theravada perspective:
There are Pali suttas where the Buddha recounts how he was this or that deity, or a lord of Hell, or even Mara (Death), IIRC.
For example:
This section of the Itivuttaka where the Buddha enumerates some divine births of his.
Or
MN 50, where Moggalana tells Mara how he himself (i.e. Moggalana) had been born as "Mara Dusi" at one time.
The implication being, that every being has been all of these, since there is no discernible beginning of this round of birth and death.
So, different mythology. The Buddha was a great being, not because he was an incarnate deity, but because he had perfected all these perfections, and yes, he had been born as a deity countless times before his last birth, which was as a human being.
So much for the mythology.
What does the "Buddha as a human" ideal do for you? Does it inspire you to practice well? Or does the idea of an avatar do more for you? Whatever the ideal is, as long as it fuels your practice, it's probably worth your respect.
At least, that's how I handle these ancient myths.
Cheers,
Florian
(edited to add sutta references)