I agree entirely about other opportunities! Any suggestions (other than our local slug)
One alternative could be for all such creatures being somewhat organized, and thus being able to talk to and treat diplomatically with all of them. Even if I can't get them to join me, maybe I could give them something in return for them protecting a border, or for providing LoS. I particularly like the idea of being able to use large forests as guarded borders by paying the inhabitants (whether with gold, magic, or some other resources or service) to harass any of my enemies that pass through it. Basically they'd remain independent but would render you some service in exchange for something else - you could even threaten them.
Also, maybe some critters could slowly restore life to blasted lands if left alone. That could provide a real incentive for not killing even some hostile critters, and for not recruiting as many of them as you can.
@pigeonx2: I remember him talking about fantastic creatures in general being a big deal, before dragons were mentioned. But I guess we'll see. (I personally think that Ents would cave to even 1 dragon. But I'm a bigger fan of dragons being the most powerful creatures around. The near indestructable forces I imagine from stories like "the wizard of Earthsea". The dragon's stats in Burning Wheel also reflect this, being unable to even be injured except by "grey" stats and weapons)
Well yeah. I guess it's just a matter of the magnitude of the "big deal." I don't see a yeti, for example, or an Ent taking on a legion of 300 decent troops all by themselves. Alone they should be capable of taking on double digits of regular units, maybe yetis on the smaller side and Ents on the higher side. But supported by an army they could be significantly more dangerous. I totally agree with you that a single dragon should be more than capable of beating a good number of Ents. The only individuals that should be able to rival dragons IMO are the most powerful late-game essence hoarding channelers and possible some other miscellaneous fantastical creatures (Frogboy hinted at there being other insanely powerful units that you might not even encounter in the game for weeks).