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:
Here is a scene out of the movie that fits almost perfect into Xsyon minus the gas powered vehicle:
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.