Harpies live in Nighon. And everything is quite clear. The Dungeon lineup consists of magically bred (manticores, minotaurs, beholders) and tamed (dragons, harpies, medusas, troglodytes) creatures in the armies of Warlocks and the Underlords of Nighon.
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 did not have just one race; there were Krigans ("devils" at level 7) and the creatures that served and were bred by them.
Well, regarding the tamed creatures (I don't like the campaign because it restricts freedom of action – this is my personal opinion, so I don't know the Heroes world very well), but I don't think they were tamed like dogs. Especially since there are troglodyte heroes.
And what about the swamp? Is it the same story?
It's a pity that there won't be "swamps" in the sixth game. I would like to see these guys there first.