Future expansions.

Harpies inhabit Nighon. And everything is quite clear. The lineup of the Dungeon includes magically bred (manticores, minotaurs, beholders) and tamed (dragons, harpies, medusas, troglodytes) creatures in the armies of Warlocks and the Underlords of Nighon.

The Rampart is indeed not an elven city; it is a city representing forest creatures. That is why there are tamed centaurs there (who usually live in wild tribes).

Inferno had more than one race; there were Krigans ("devils" on the seventh level) and the creatures serving and bred by them.