Flying scimitars spawn with rare scimitars. Valks, Shadow Masters and Shadow Warriors also spawn with equipment. And apparently the ancients also have equipment, though I don't know what; that's not in monequip.txt.
I think (but I'm not sure) that the items they spawn with are unrelated to what they drop. So if Bishibosh spawned with that, you still wouldn't get it when he died, but the fact that they were already spawned on him would mean that you can't find any other of those uniques again in that game.
EDIT: I killed Blood Raven and didn't get a rare short bow. Also, flying scimitars can drop the exception version in nightmare and the elite in hell, while they only equip the normal version. So it seems I was right.
I think (but I'm not sure) that the items they spawn with are unrelated to what they drop. So if Bishibosh spawned with that, you still wouldn't get it when he died, but the fact that they were already spawned on him would mean that you can't find any other of those uniques again in that game.
EDIT: I killed Blood Raven and didn't get a rare short bow. Also, flying scimitars can drop the exception version in nightmare and the elite in hell, while they only equip the normal version. So it seems I was right.