Since animals and "barbarians" are being sepearted in function (and through a mod, barbs can actually hunt animals) it leads to some new interesting early and mid game fun.
I have some ideas on this. Specifically, I think it'd be awesome to see the "non-civ" units divided even more.
Barbarians (a rival non-controlled civ), Animals (as they are now), Beasts (animals that adhere to specific terrain/climate), and finally "Monsters" (Your garden variety "I dont know where this unit goes" unit.
Barbarians would function as they do now. Getting free units, building cities, constantly razing/capturing your cities and ravaging your country side. As per normal wonderful barbarian behaviors.
Animals would function as they do now. Spawning, harassing your explorers and expansion plans. Also capturable to fit in with the FFH world.
Beasts. These buggers you could arrange to have ONLY travel in certain areas. This could be Giant spiders among others.
The giant spider may only be allowed in Jungle, Forests, and similar features. Perhaps they'd be given the potential of getting access to new terrains by promotion. "Pit spider" evolution/promotion would allow them to travel on deserts, and plains. "Crag spider" may allow them to traverse hills and mountains (a definate advantage). Each "beast" would be limited to certain terrain - but would be very deadly in those terrains, and whatever "den" is created would be even more deadly. The "good side" is that perhaps some sort of reward is given for eliminating beasts. Perhaps "gold from hides" would be a good boon. Or den's might become very rare and very potent luxury resource tiles when destroyed. This allowes for myriad beasts of D&D lore - AND if myriad beasts were encoded into the game, it might be rare to see any one kind of them. An "Ice Wyrm" found only in glaciar spots, might never appear for 20 games, if there were other compeditors for spawning and terrain type. So seeing one, and slaying it would be all the more fun.
Finally,
Monsters. I see monsters as being the "catch" all for fun, unique, and special units that the team will most likely develop into questing and other fun things for scenario's later.
Skeletons and many summons would be "monsters". Each individual monster would obey its own unique rules. Skeletons would have many animal, and potentially "bestial" and "barbarian" traits. They can spawn anywhere, they're not animals, but they can have "spawn point/dens".
Monster may also be a nice catagory for Dragons. Unless they get their own.
Just my thoughts.
-Qes
ADD: Potential Ideas for "beasts"