I don't know how I managed it, but somehow when I was constructing a staircase, I left a big hole in the floor that opened up into the top of a cavern, and didn't notice. The only real impact was that from time to time a bugbat would fly up and wander in, and I would find a loose bugbat mucking around in the lower levels of my fortress.
They were down in the wild and wooly cavern-adjacent part of the fort, with the metal shop and animal cages, and they didn't even seem hostile, but I still didn't want them pestering my dwarves. I set up some cage traps, and by the time I'd figured out what the staircase issue was and fixed the hole, I had a bunch of bugbats in cages.
So, what's good to do when you have some stuff on hand you're not familiar with? Start playing with it and see what it can do. I set up a room for the bugbats, started taming and training them, and learned that apparently what they can do is fuck, because in very short order I had an absolute shitload of bugbats.
A little while after that, I had more than that. Way too many. If I had had sense, I would have just opened another hole into the cavern for them and let them rejoin their natural habitat at this point, but I guess I felt responsible for them or something, because I kept training and breeding them long after it had become clear that (a) they were useless, and (b) I had more of them than I could ever conceivably need for any purpose, even if they had had a purpose. I started slaughtering them, trying to get a handle on things. All that happened then was they got loose. They would escape when some dwarf that was hauling them for slaughter would get distracted, or one would leave the room when the door opened to take out another. They started fucking and making unsupervised pups out in the main fortress. Bugbat pups became a frequent feature of my halls and stairwells.
Things were busy and they were slippery and numerous, and it was hard to make time for the level of attention it would have taken to really address the issue, and they were pretty much harmless, so they stayed as unwelcome but tolerated guests. But over time they became a menace. One of them had an altercation with one of the fortress dogs and injured him, which pissed me off. And then, there was an incident when the poor bastard who was assigned to train the goddamned things had some sort of bad interaction with one of them, and tried to abandon his task and leave the bugbat room, but the room was so stuffed full with upper and lower case "b"s that he couldn't manage to push his way through them to the door, and I thought through the screen I could feel his rising panic as he realized how badly outnumbered he was, and that some of them were barely trained, half wild, and that the tenor of the room had changed and he was totally alone, and tried to control his terror as he struggled harder and harder to reach the door through the crush, before they all fell on him at once.
I decided to kill them all. It took -- no joke -- many years between the firm decision, to when it actually came true. The issue was that there were so many that it was impossible to give an order that would apply to all of them, which could be carried out in full before they had made more pups and created a population to which the order didn't apply. It was tedious and difficult to almost a mind-numbing level to even find them all, or issue any order on all of them, never mind the time involved in actually carrying it out, or the new ones that would arrive in the meantime. I built two butcher shops and assigned multiple dwarves to full time bugbat-killing duty, severely annoyed that my labor force was having to make this a full-on fortress priority instead of some more productive thing they could have been working on, but the time for taking the bugbat issue lightly had come to an end.
As with so many things, the end came a little anticlimactically. I morosely went to pore over the list of bugbats and re-designate the new pups for slaughter as I had done so, so many times before, and found no bugbats. I found myself like a prisoner who's been set free, disoriented and blinking in the sun. What do you mean, no bugbats? My dwarves can get back to work now? No rotting bugbat corpses in the butcher shop because someone was too busy to take care of it in time? No lower case "b"s blocking the door I need to close? What the fuck do you mean there aren't any bugbats?
It didn't even make me happy. I think I was still too irritated about the whole debacle to even reassign their keepers to any other duties right away. I simply didn't want to deal with it. No bugbats. Great. Thanks. Wonderful. Can I go now?