Hello,
With the Ruby Knight gate having Perimeter Prominade II at depth 19, I thought I'd try my luck at finding a Sloom. As of yet, I've not found one, and I've done quite a few box openings.
However, I found something stranger..
http://wiki.spiralknights.com/Exploration#Monster_Cages
As you can see, Mecha Knights are not an enemy that you can find here, but I've found 2 now. It's not a good sample by any means, but I've found 2, whereas so far I've found 0 zombies, lichens, silkwings, gremlin scorchers, demos and understandably sloom and love puppies.
What's even stranger, perhaps, is that this Mechaknight doesn't seem 'normal'. You wont be able to see it in the following pictures (but shown here for proof), but the charge attack of these mechaknights shoots electric bullets MUCH MUCH faster. Sadly, I only took screenshots of 1 (I didn't realise it was undocumented previously), and I have no such means to record
http://img819.imageshack.us/img819/6517/mechaknight.png
http://img718.imageshack.us/img718/5841/mechaknight2.png
Whilst this gate is here, I will keep trying to find a sloom, and perhaps there's more undiscovered monsters that I'll find?
I posted this here in case anyone wanted to update the Wiki to make room for this Mecha Knight found here, or whether you wanted to research further these bizarre mecha knight behaviours (who knows, I might have been lagging in a weird way for it to 'speed up').
EDIT: The next time I went down I found a Devilite (first one!) and a Blast Cube (also undocumented). BIZARRE.
Strange. I've actually freed all the possible Beasts and Gun Puppies that have been recorded to be found in Monster Cages including a Love Puppy though I might not have found a Red Rover inside. I can't quite remember.