Renegade Brilla disguised herself as a Mammoth?
I accepted the quest to kill the renegade Brilla (don't remember if that was her exact name, but since names are randomly generated, I don't think it matters all that much),
and when I found her, she was 'disguised' as a Mammoth Hulk... which attacked and otherwise behaved just like a normal Hulk would, but it still used the "pain" sounds of a female human character whenever I hit it. When it died, the Hulk's death animation played out, but the corpse was instantly replaced by an alive and idle but otherwise inactive Hulk named "Brilla's corpse" that slowly faded away after looting it.
I could kick myself for not taking any screenshots...