You fight with what you have but then sometimes you lose to someone who has a better army than you do and disappear from history. So in the long run you end up fighting with what actually works in your terrain. And I think it's very interesting that horses, who are only really indigenous to the steppe, ended up proliferating over the entire earth in a way that elephants did not. Hundreds of elephants on the battlefield sounds great if you can feed them though!