The very first time I setup an arena and tried to pit a creature into it. I looked at my vast collection from sieges and caverns, and choose the giant mole rat. It looked the least dangerous out of everything. A peasant came, pulled it out of the cage, and the mother fucker bit his head clean off. The bastard and his new name proceeded to run all over the fort causing no end of trouble until I locked him in the managers office. He stayed there with a mortified manager until axedorf militia humane animal control arrived.
In my latest map, which is freezing/cold, I was moving everything inside asap into what would eventually become the main hall. I had all the food, necessary rooms, and workshops setup, and i went out to move my 'everything else' stockpile, where the remaining crap from embark was, and I saw a monkey. Then I saw his 8 friends. I've had monkeys steal things from me before, but I've never been gangbanged by a whole mob like that. I lost every single piece of cloth and thread I brought with me. Shortly there after a miner went on a pick in the brainpan murdering spree because he couldn't find any thread, which is exactly why I brought it in the first place!
Since then it seems like after every ambush, those god damn monkeys show up and take the best spoils from the corpses before I can get my dwarves out to collect it. It's like they think they deserve a share of the loot now. One day monkeys...one day.