I know it has been mentioned the possiblity of mutants and folklore in game. How ever i would like to suggest something like a giant mutant bear from the 1979 movie Prophecy:
Thats the kinda scene i would like to play out in Xsyon, If you look carefully the teepee in that movie looks awfully familiar.
Look at that thing, if you saw something like that coming out of the green mist i would run, maybe try to kill it and then run when i got knocked to 50% health in bone armor.
I think we need some epic/legendary unique animals IE theres only one of them and if its killed it goes away for a season, something that is impossible for 1 person to take down at max skill, maybe even 3 people at max skill if they didn't get hit.
The legendary system was supposed to work where if an animal wins in a battle, whether it be with another animal or a player, it gets stronger. And with each battle it sort of "levels" up. So it would become harder and harder to kill until it becomes a legendary monster where it would take a LOT more than one person to kill. That is what I think the system was to work, and I believe that the legendary monster would be bigger and probably have distinguishing marks. I could be wrong though. =P