I feel like a semi-passive monster could be some sort of leech (or have the mechanic I'm about to tell)
Laech Monster (WIP name)
Perhaps looking like a lady bug mixed with a leech; The Laech is attacted to monsters harboring a lot of juice. They would latch on to elite monsters and hydrandgla, leeching off their juice and causing their attacks to slow or have less power. They would drop "the good juice" after you kill them (when they are in a full state of course). They could also hassle the player by making their level bonuses go down or be nulled.