Very much like how we have rare monster spawns such as love puppies and soul jellies, perhaps we should have a random chance for a boss we come up against to be altered slightly.
All boss monsters would be standard on difficulties lower than Elite, and on elite every time a boss is encountered it has a rare chance to have an alteration that may change a player's tactic. These bosses are conceptualized below:
Snarbolax
The supreme stalker of the gloaming wildwoods has company with other beasts. Now we get to meet them!
Vog Cub and/or Skolver