Have you tried rendering 3D characters into Doom-sized sprites and always end up with grainy, fragmented and pixelated (in a bad way) results?
That is because realistically-proportioned models do not translate very well into a low res space. What you need to do is re-model your character/monster to have a very distorted, squat-like proportion.
The easiest way to do this in Blender is to pick your favourite Doom sprite, then add it to your 3D viewport as a bacgrkound image:
1) Press N, a new side panel to the right of the screen should appear
2) Scroll down the side panel until you see "Background Images". It is usually unchecked and is collapsed. Click it to expand
3) Click Add Image, and then Open. Choose your Doom sprite.
4) Position it in the canvas to match your model
5) Start remodeling!
You'll want to do both a front and side view to make sure your modeling is accurate in 3 dimensions.
Here's an example of what I did, and what the resulting renders can look like. Your model will look stupid in high resolution 3D, but when rendered into 50 - 60 pixels, it will look awesome. Pretty neat, eh?!
(Click for larger version)