Just don't give them the ability to swim? We don't have to be 100% realistic.

Ok, and if they swim on the pond?  You could hunt them with ranged weapons but you cannot collect the prey afterwards, right? (unless using a fishing boat which might be an overkill). Not yet really convinced. I mean, for eye candy purposes it is certainly nice to have even more variety in both flora and fauna. But someone needs to create the animations and skins. In the past there were already quite a few animal modeling tasks...

I think the real question isn't "Why are massed Champion cavalry so broken?" The real question is: "Why did I let my opponent mass expensive Champion cavalry in total peace and quiet of his turtled-up base?"

It's not the same. If your border ally is harassed by camels, you can counter it by your own cav (you need less then camels) and having the speed tech makes a big difference. It's unlike another buff of hp/damage.

Maybe near the lakes and ponds? But, it would be just a re-skinned chicken anyway.

Eh. 10% isn’t that much in the cav vs one discussion—cav is already much faster than inf. Getting the Brit speed hero doesn’t suddenly let make inf much better than cav with the tech, for example. Functionally, the tech means you can raid other side of map for an extra second or two before returning to your side to fight inf. That extra second or two doesn’t change that much.  You also see people forget speed tech and still become dominant because they massed champ cav. That shows there is more going on than the speed tech.  the cav vs spear example you give isn’t representative of real fights. In real fights, you have a bunch of range killing that spear too. It’s also pretty common that you see a player get a “good exchange on res” with spear vs cav but can’t keep up fight because their pop dropped so much and the backing enemy range units are able to overrun the remaining base   honestly, haven’t seen any difference with the popularity of sele or Persia champ cav. Sele and Persia have both always been relatively unpopular (esp compared to the civs you mention) bc Persia/sele are slow civs. Persia is just played a little more now because people figured out that immortals are good.  

Good point, but would it make much difference to have ducks rather than chicken (or ducks and chicken), except "eye candy"  - or is it more historically relevant? Might be preferred in some biomes?