Bull Demons are one of a class of monsters normally called Demons in the games.
Doom/Doom II RPG
In Doom RPG they are the weakest form of Demons, and just a light shaded pallete swap of the "Pinky".
In Doom II RPG the Bull Demons are special case, and not just a 'pallette' swap of the regular Demon graphics. They are tiny about half the size of regular Demons. When they are first introduced, a character points out that they look 'cute' and thinks its harmless until it head butts him down an elevator shaft. It is discovered they originate from the Pinkinator a giant worm like creature.