We mixed a lot of enemies from all doom games because there were so many cool demons and monsters that we couldn´t just stay only on the first two games. Also we think the concept of “demaking” a game is very interesting. Reversing aspects of, for example a cacodemon to make it cute or grabbing an ultra detailed and scary character like a Summoner and make it 60 pixels tall was endlessly entertaining.
Of course, while making all this sprites I always had theirs very-hi-res counterparts near by. I actually bought the Doom 4 art book only to have a legit reference guide to make my silly pixelart.
I used the concept art for everything from colors to ides for repeating tiles for backgrounds and floors.
And here are some examples of the current sprites on MiniDoom 2 vs their original 3d hi-res inspirations: