We are half way. Tanks a lot SD for the dungeons! Now we need that multi tile hero, or just some little dragon to pet.
So pets would be awesome, I am all over that idea. Maybe minimal combat abilities but slight utility here and there?
But I digress, back to the original post. I believe it would be really bloody awesome to have 'monster' heroes. As already mentioned, darklings is a good and easy start.
My contribution to the other idea, of per say, wolves would be to have a werewolf hero, it would be fairly easy to implement a quest where a normal person goes missing and you find them and they are a permanent werewolf. After accepting them into your party they could wear some of the standard armors, although only some. Use melee weapons (or fight with claws), and finally have abilities like lunge and blood-hunt. Of course, no horses or magic for these wolf men.
On another note, how about making that troll that you meet who asks you to help him be an actually 'henchmen' type. Maybe men are unwilling to call him their direct leader but he could be something of the sort. Being able to use weapons he finds, equip some armor, etc..
The same goes for 'Crow Demons' or other magical creatures, why not have one in a thousand that will have a mind of its own? A background? Some armor, a different weapon, etc.
I would also add, that I believe it should be difficult and unusual for a player to acquire these heroes. As they would probably be quite unique themselves there would be only a handful and as such should be things that are hard to obtain.