So I've missed this every playthrough until now, out of sheer accidental luck, lol.

It's the http://images4.wikia.nocookie.net/__cb20110717202412/doom/images/6/6d/Cherub_render.png shown on a wall in RAGE:

-http://img841.imageshack.us/img841/5621/cherubinrage.jpg from Rage


To me that says it will definitely be in Doom4
I liked the Cherub, even if it made me question my morality and killing babies... I got used to it, lol, so I had no problems killing the babies in Dead Space.
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EDIT: We have also seen now what is either an Arachnotron, or the Spider Mastermind:
http://imageshack.us/photo/my-images/80/shot99998.jpg/

Hey! Haven't noticed before.
Must be one of the staircases in Dead City, right?

Also had some problems with shooting the "babies" in Doom 3 at first.
Until they appeared in large quantities, permanently jumping into my face!
That made me so angry that all my moral objections were blown away in seconds. Same with the babies...

The introduction sequence of the bloated pus babies in Dead Space 2 with that exploding mum was so over the top, i actually had to laugh...

I hate those things. Small, quick, hard to hit, in your face, and come in packs

Cool, never seen that one before. It does seem to have a goatee though.

Lol, dude that was the first thing that came to my mind when I saw it... I was like, "so the cherubs have facial hair?" xD
Yep, I was just looking at the Maledict picture, which is impossible to miss, and then when I turned around I was just... idk I felt it was strange I never saw the Cherub before. But there it was.
And yeah, I totally had the same experience regarding the baby monsters, including the fatties in Dead Space xD
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I should mention, these hidden concept arts are very hard to spot, and my screenshot was much darker, I raised the brightness and contrast of the image.

As you asked, Vynzent:) Here it is:
http://imageshack.us/photo/my-images/80/shot99998.jpg/
Was found in dead city in an open area near mixom building covered with red stuff))
Along with Vynzent's discovery we now have 2 or more demons wich may show up in Doom 4:)