Well, there are Shadow Druids (who're technically evil in 2E and definitely evil in 3E), but I'm not sure if the group existed at the time of Myth Drannor or not. Even if they did, they likely wouldn't side with either group.
Incidentally, the drow mentioned above allied themselves with a group of soldiers from Myth Drannor, albeit rather briefly and only because both sides had no other choice (the Army of Darkness threatened a Drow stronghold). It was apparently the last time surface elves and drow ever consciously worked together (not counting the odd rogue like Drizzt or Irenicus) in the Forgotten Realms.
The druids, satyrs, treeants, centaurs and spelljamming ship were on the side of Myth Drannor, but not for the the entire war (either they got killed off early, or they only took part in one or two battles). The phaerimm and banshees fought against the Army of Darkness, but not with the forces of Myth Drannor (i.e. they were enemies of one and neutral towards rather than allied with the other).
The Army of Darkness did have a couple of evil dragons at their service, but the other evil dragons that got involved were either awakened by all the fighting or simply took advantage of the situation - either way they gathered up "food" from both sides. The Cult of Moander were merely lurking around the place, and didn't really get involved in any of the battles - though a scirmish betwee the Cult and a group of knights from Myth Drannor did form a sort fo "prelude" to the war.