What's the best use for a trained dragon? :3
Lunch.
Yes, really. Just butcher the poor thing and call it even. I've tamed a few dragons and then spent the effort to war train 'em and they've universally been more trouble than anything else. Breathing fire when you don't want it. Not breathing fire with you do want it. And so forth and so on. For example:
Dragon vs Forgotten beast... draw. Half the time, the dragon won, other half FB won. Every time, dragon comes out severely injured.
Place dragon behind fortification to encourage breath use and protect against melee... Dragon just sits there looking stupid.
Pasture dragon near "welcome center" in order to at least give kobolds a warm welcome. Managed to toast one kobold. Goblin siege appears. Dragon charges into welcome center, gets slaughtered by goblin squad.
Pasture dragon on single tile surrounded by a channeled vertical sided moat (so silly thing won't go charging and simply use it's breath weapon). This sorta worked. Yes, the dragon breathed fire quite often when enemies were near. Had to replace a few hatch covers in welcome center, then build "shield wall" to protect newly installed hatch covers from dragon fire.
Save yourself grief and frustration. Dragon burgers is the way to go.