Nigon is the second underground race.

Come on, why stick to a template? If there's an original idea for a unit, but it doesn't fit the overall concept (like, a flying unit is required, but if it's the only one, it needs to be at least level 6), then who cares? Just create the unit, and you'll be happy. It's just that the Ifrit is cool and fits, and the Harpy is a flying unit.
So, in principle, this discussion is pointless.