Sup guys, I just started doing Black temple last week to try and get the Warglaives and some transmog, and I noticed some cages above the soul pits, I logged on my hunter and eagle eye'd as much as I could to see what they were, to my astonishment I found that they were actually caged prisoners within the Black Temple instance itself, there's a female orc with a Thrallmar Tabard on, a male draenei with a Shattered Sun tabard, and a Wildhammer Dwarf.
You can't target these mobs and can only see them through farsight / eagle eye, but I thought it was really cool, one of the dwarves is actually sitting on the skeleton of what looks to be an orc or draenei skeleton.
Here is one of the pictures I took