The Demons evolve based on their past invasions , few examples
Imps - Due to their slow movement they are incredibly easy to gun down, eventually they evolve to be more mobile eventually becoming nimble but slightly weaker than before.
Pinkies - Strong, but easy to kill by pumping rockets or super shotgunning em, so they gained armor plating in the front, invulnerable to most bullet attacks, but has a small weakness at the back
Barons And Hell Knights - Similar to the imp, more nimble, and also stronger, much more aggressive than the slower fellows they once were
Doom 64 demons are mutated off shoots