Future expansions.

magically bred minotaurs

Um, minotaurs? Why? Aren't they a separate, original race?

Inferno didn't have just one race; it had Krigans ("devils" on the seventh level) and the creatures serving and bred by them.

Again, not all of them were bred, and this is mentioned in the fourth game. Ifrits were independent; they simply allied themselves with a powerful "ally."

As for the centaurs in the Dungeon and the Fortress, we shouldn't forget that Antagarich and Enroth are two different continents, and it's not surprising that the preferences of those living on them may differ, even though it's the same race.