It's commonly accepted fanon, is all. And I don't get why you accept Dr. Hooves being the dad, but discount Derpy as the mom. Neither of those have any suggestions from the show, so why is one right but the other wrong? Is there a certain reason she can't be the mom? And Baby cakes already showed us, with a pegasus and unicorn baby coming from an earth pony couple, that the actual parents don't actually have to be the same race as long as somewhere down the line there was cross-breeding.