Actually, the "ooze monsters" were also parasites, but instead of trying to lop your head off, they would opt to suck your organs out mouth to neck by popping out of the victim's head like some sort of demented pez dispenser, and were pretty much adapted to feeding directly from its prey while growing in size to match the host's original mass. This is in contrast to the plagas which basically hid in their hosts while manipulating the body to sustain themselves, and generally weren't meant to survive once exposed.
It was unclear how some of these things were still talking, or had anything remaining of their brains given their altered body structure...also they had this strange ability to determine whether or not a gun was aimed at them, and then alter their gait or posture as to avoid being shot in the head if you took too long to aim.