I managed to capture a dragon some seasons ago in my current fort. Having applied some training to it, I left it in its cage, not wanting to risk the potential !!fun!! that might result before my dorfs can properly control the beast, with the hopes that the trainers would rack up experience with dragons periodically training it in its cage.
Recently, however, I noticed that when I got the alert that the Dragon had become semi-wild, it was no longer in the animal training list at all. Attempting to rectify this, I tried transferring it to another cage, as well as luring it into another cage trap after remote-deconstruction of its cage, in the hopes it'd show up in the list again as 'not tame'.
However, I failed to understand that semi-wild creatures are still technically tame and aren't subject to traps, which the dragon happily skipped over into the main fort area before deciding to turn wild. I have not unpaused the game yet, wanting to at least understand why it didn't appear in the list. I can only hope that casualties will be limited and that with the cage traps lining the stairwells and chokepoints I can restrain the dragon again.
So my question is, why would the dragon have disappeared from the animal training list? Is this a known issue? I've looked for myself and haven't been able to dig up anything.