Engorged Mooks appear to be Undead Mooks who have had their stomachs cut open, stuffed with explosives, and crudely stapled shut. They are very common in the Hell section of the campaign.
Engorged Mooks behave as suicidal mooks do, in that they charge directly at the player upon detecting him/her. They differ from the suicidal mooks, though, in that they will fling themselves through the air, doing multiple somersaults in their zealous fervor to murder the player brutally. They can also be spawned by warlocks continually until the player kills the warlock.