I had a weaponsmith make me a steel spear, which I displayed in my dining hall via weapon trap to make dinners that much more enjoyable. Okay, that doesn't sound very useful.
But! A dragon came unto my, stumbled into my *Serrated Green Glass Disc* hallway, lost all it's teeth, got its scales dented, received a concussion, and started writhing in agony on the floor. In the only entrance/exit to my fort. Then came goblins, which the dragon promptly vaporized. Then a were-man (I forget which variety) tried to attack, but suffered the same result.
I realized that my fortress was in need of a champion if it was to survive with such a fearsome (albeit severely damaged) enemy at the gates. So I ordered the great spear taken down and given to the most talented spearwielder among my dwarves and sent him alone to face the waiting beast. He snuck up on the lizard in its sleep and, after a few false starts, managed to skewer his foe through the brain.
And thus the tale of the famous spear and it's anonymous bearer is told.
The thing saves you from everything else that tries to get you, and you kill it?
If you couldn't tame it, you should at least have weaponised it -- devise a means for it to guard your entrance at all times except when you specifically want dwarves to get out, or create an alternate entrance that's twice as long but put a restricted traffic zone on the short entrance so gobbos and the like try to get past the dragon but dwarves don't, or
something. There's gotta be something. I mean seriously, a dragon inadvertantly guarding your fort?
Ah, well, I'm probably just jealous it didn't happen to me.