Some years ago in the fortress of Sparkrazor, I pissed off the elves badly enough for them to send some 30+ simultaneous ambush groups. Seems they didn't approve of my cutting down every tree on the map and then shooting their diplomat in the face for several years in a row. That spring my game was being interrupted by messages of "Curse all friends of nature" non-stop for a month. Interestingly, they all appeared in basically the same spot on the map edge.
I was well prepared for them, and was able to funnel the entire group of some hundred or so elves and their mounts onto a retracting bridge over a pit which was deep enough to kill some of them and massively injure the rest. The pit, now packed full of broken elves crawling in agony over the mangled bodies of their companions, had a wall lined with fortifications several levels above the floor. Not wanting to put perfectly good targets to waste, I stationed my marksdwarf Captain of the Guard at the fortifications with a large stockpile of bolts. He then proceeded to spend months picking off injured elves from above. He's a Legendary marksdwarf, but there were a lot of elves and mounts in the pit. It took him long enough to pick them off that one of the horses the elves brought gave birth to a female foal.
Thanks to a weird bug in DF, that foal was hostile to the elves. She immediately began fighting the horribly injured and crippled prisoners trapped in the pit with it. She must have scored a kill, as it gained the name of Ofalini, "Foldedcats". She also gained a remarkable list of injuries: broken lower and upper spine, broken nose and ribs, mangled left eye, missing right ear and right rear hoof, and pages of bruises and cuts. Eventually, I opened the pit into a corridor lined with cage traps, capturing Ofalini and a few remaining invaders.
Ofalini the horribly injured foal, child of elven invaders, born in a blood-soaked pit of corpses and death and knowing only violence and pain, has become somewhat of a mascot in my fortress. I didn't have the heart to butcher her, so I tamed her and gave her her own pasture on the surface. Somehow, she didn't die of infection or blood loss, and healed her injuries as well as possible. She still can't walk, and is probably deaf on one side and blind on the other, and her body is a mass of scars. Yet she manages to drag herself around her pasture and eat enough grass to avoid starving, constantly flashing a red plus sign.
Ofalini, now having grown to become an adult horse, surprised me by giving birth this year. Impregnated by spores from the latest elven invasion, I suspect, and her massively injured state didn't prevent her from giving birth. The foal she gave birth to had the status of 'tame', yet every dwarf who saw it ran away terrified as if it was a hostile invader. Fortunately, the foal also ran away from my dwarves. I wasn't able to order my military to kill it, but they were smart enough to try and kill it on their own when I ordered them to a nearby position.
I've been trying to train up some more military, especially one novice Hammerdwarf who I've given an artifact steel warhammer my one weaponsmith recently created. I hoped to use the foal for a bit of live training. At first he and the other novices couldn't even catch it, chasing it around the map but never getting close enough to land a blow. I eventually called out my Captain of the Guard, who put a few crossbow bolts into the foal, enough to slow it down so the others could catch it. The novice Hammerer caught up to it, and then proceeded to spend some time whacking it with his artifact hammer to almost no effect. The combat log was full of page after page of him inflicting bruises and fractures, but not seeming to accomplish much. Eventually my Captain must have decided this was enough, casually strolled over to the foal, and with a single blow from his steel crossbow crushed the foal's skull, killing it. So much for training the new guys. Do steel warhammers just suck, or is my novice Hammerdwarf just no good?