Well, as wargs and horses are different resources, I guess it would require a button upgrade for each one.
Yes, mounted units are faster, better,...and if you have footmen with high level, it would be nice to give them a mount. Or to the many foot troops because could not find mounts in early/mid game. I don't find it necessary, but if it is implemented, it is wellcome.
Talking about mounts, I would like to give a bear mount to a juggernaut 