Since the game is going to be 99% land/air based (As they've said it's a 'land' strategy game) I'm going to leave out water ideas for now.  Also good ideas for animal/monster/special powers/attributes would be good. 

 (Im at work, am tired, and cant spell)

Pegasi (If that's how you plural them):  These might be in already but if not have many good reasons to add them to the game. 

1-They're cool flying horses.  2- Putting armored soldiers with lances, Joust anyone? 3. Flying Archers, aka Super Mongols.

Shadow Mares (aka corrupted unicorns) 

1. What evil lord doesn't want to ride in to battle on a magical, flesh eating, horse?  2.  Flesh eating horse!  3. Only real way to make traditional fantasy Evil Knights.

Archons, aka lordly angel warriors.  Probably in, but I can't remember if they were in MoM or not? 

Were-creatures, wolves, tigers, whatever.  All sorts of interesting possibilites with Were-creatures.

Vorpal bunnies, just because. 

Djinn, aka Genies, -usualy evil creatures that protect certain areas. (These WERE in MoM)

Ancient race, there's talk of the old race that used the dragons, and other creatures, as pawns.  Would be cool to have very rare encounters with them.

...ok this thread is a dud, but I'm bored at work- someone else come up with GOOD, UNIQUE, creature ideas!

NOTE: Ok, one semi decent idea in another vein.  Spell that combines two units/creatures.  The artwork would be a problem so you just get one type of art for whatever you combine, BUT the stats are randomly jiggered between the two units so you might get a super army or total slime (And the middle ground of course)- The peril is of course that you lose 1/2 of what you're putting together.  Say if you put a dragon and a troll together you only get one being after.  Or if you put 500 swordsmen together with a sheep, you only get 250 men-sheep after.  (you would get to customize name of unit of course).  Just a percentage roll depending on your spell/skill level and any other help/detriments to the spell would say how well the meld went.  This way you could create almost anything you wanted in game, even if it wasn't modded in.  Lack of artwork would sort of suck but I for one would still love this (Obviously).