It seems like bears and wolves would both be suitable, after a quick check bears were essentially hunted / exterminated in the 17th / 18th century (except for a small pocket) and wolves were roaming around in the 19th century.
Someone can correct me if I'm wrong. So, yep, bears and wolves would fit in, and add quite a bit. I'd also recommend wild boars that can charge and gore you.
In terms of actually adding these in - wolves should be pretty easy, add a new model for the dog and reuse the animations (although modified animations would make them look unique to dogs).
Adding bears would be quite a lot harder - adding in a whole lot of animations for the bear, and if you wanted it to look good, Henry would need a couple animations for getting clawed.
You might be able to reuse the dog animations applied to different skeleton, but it would probably be super glitchy (especially where the limbs meet the body).
You would also be missing a lot things that make a bear look like a bear - like an animation for it to stand up and growl.
Not sure how hard it is adding new animations - especially to animals, I've done a heap of animations in the past for first and third person humans for Cryengine 2 and 3.
However, it is likely that Kingdom Come is different and I'm not even sure if it uses same type of .caf and .cal files for animations.
The other thing would be the AI - using the dog AI as a starting point should work. However, to get wolves / stray dogs to act like actual packs some group AI would need to be set up, or at least something that gives the appearance of group AI - obvious thing would be having the wolves circle enemies. This could probably be done without requiring the AI to communicate to one another - although it would probably result in clipping. The harder thing would be having packs co-ordinate attacks as a group, (like wolves in Zelda BotW), but this probably isn't necessary since dogs already tend to kick Henry's ass without pack tactics.