Nigon is the second underground race.

If we're talking about trolls, then only cave trolls, like in Tolkien's works. Big ones (just like in the movie). Then, the upgrade could be one type of troll in armor, with a helmet, and another with a bone sticking out. Overall, it would be cool. And hippos have no business in caves! They love air and barbarians too... until they get hungry. And it's not a good idea to use them to plug up a hole in the castle!