On my first draw of the game, I drew a Golden Nymph. There is no Golden Nymph in my deck. I don't even own one.
I was using a stock CCYB deck. I was playing against Destiny. But since it was my first draw, I hadn't yet had the opportunity to play any cards. So it is not possible to be a mutation/rewind combination, or some such.
Sorry, I should have captured a screenshot or something. I will if it occurs again.