Well they are supposed to appear in all the anti virtue dungeons, so I would imagine there could be more tho it appears noone has found any others yet other than in despise, tho the despise one took a while to appear so not sure if they are 'staggered' nor if there is a respawn.
Other than the box, the only other 'loot' is gems. One pile of citrines, rubies and amathysts are 'normal' but the other piles won't stack on normal gem piles, one pile of rubies has a slightly orange hue to them, and the amber stacks on normal amber.
From what others have written, it is best to use a dread warhorse if you want your pet to remain 'happy' while it fights, I also brought in a mare, which dropped to rather happy pretty quick, so if you are using another pet watch its morale.
Even tho when there alone the warhorse died a couple of times, sneaky and I didn't die at all while killing it so it isn't too nasty, however, it has a load of hitpoints and takes a while to knock it down.