I don't know, it's not a simulation. Animals are a good "tutorial" with easy fights. If they become totally dumb, as much totally remove these fights ...

Yes it would get more difficult later. 

But its not a spike. If you think thats a spike i am not sure how easy the game has to be for you. 

Having different AI behavior is interesting. If you just have the same for whole game its kind of a joke for a strategygame that has to compete with Civ6. 

I'd have to agree with this. animal AI being dumb wouldn't mean a difficulty spike when fighting anything that's not animals. animals should be seen as xp fodder, only really being a problem early on in the ancient era, before getting troops more capable of combat. animals charging at the nearest enemy makes sense. there are some animals that are capable of tactics, like wolves and such, predators that typically target the weak in a given pack to begin with, and they could reasonably use a more complex tactics AI, but deer should not. they are prey, not predators.

besides, barbarians and outlaws should appear soon after the start of the game anyway, so it's not like you'll be getting any kind of spike. animals being dumb doesn't mean humans not being dumb makes them suddenly way more difficult than if the animals weren't dumb.