Alright, get this:

I have a homebrew setting for D&D, and I made a custom list of wildlife that is present in the setting. It's a temperate area, no arctic, no tropics, no not!africa. It has elephants, but they are mammoths/mastodons with the stats of asian elephants, and I used some of the additions from frostburn animals. I call them "kogoloxen"; a name I picked up from warcraft back in like 2003; I dont remember what they are.

So far in the setting, no group has made real use of them, but I might have an npc or group of npcs use one or two or three at some point, and theres nothing stopping players from going and getting some and training them for combat. Humans use horses and the really elite knights ride gryphons. Orcs don't generally ride anything but one tribe rides bison a bit. Gnolls don't ride. The other races don't share the kogoloxens habitat.

Civilization isn't as big or widespread in my setting, it's more like the iron age or wild west in that sense. Mass combat with huge numbers of soldiers just isn't something that happens often. The bigger the group of low-level soldiers, the more likely they are to get wiped by some high CR threat.