The Bone Raiders

You don't fuck with the Rakada. The people living in the Great Grass call them the Bone Raiders, from their charming habit of displaying the bones of those they kill on their horses and armour. But being a raider is tough these days. There's a new High Chieftain ruling the Grass. He's had it with the raider clans, and plans to use his sizeable military to do something about it. And then there are the araatan: fire-breathing lizards the size of elephants - one of which happens to turn up in a cute little settlement the Rakada are in the middle of raiding.
Sayana is a Rakada scout, and in the chaos of the raid-gone-wrong, she finds herself on the back of a rampaging araatan. Whoops. In a panic, she discovers she can steer it, like you would a horse. It's frankly amazing she survives any of this. Once Sayana gets an idea into her head, it's awful hard to dislodge. And now she has a doozy: what if the Rakada could swap their horses for araatan? Train the lizards to act as mounts? That would even the odds against the High Chieftain, no?