they kind of worked on those monsters, they edited the cacolantern sprites so it didn't just look like a yellow cacodemon, dark imps were put into a variant pack, and they added the lunge for the blood demon. they would have to edit the abbadon's behavior or look before it would be accepted, for normally it is bit bland.
Actually, the blood demon "simply" has unique sprites (which aren't from skulltag). It's the mauler demon which has the leap attack (and looks quite different, again).
Anyway, back to the topic.