Only thing thats never kiled me are Wanamingos, Floaters, Centaurs and radroaches. Oh and Brahmin. How one of those guys kill you ill never know.
They wallop you around the head with their 'arms' usually with their friends. I've come very close to dying from those, but have usually been saved by my companion or a Fire Gecko who just happens along at the right time. Of course, Fire Gecko then turns on me, but I'd rather fight him than that other thing. I have Floaters turned off, so don't know what their specialty is.
I've been attacked by Mad Brahmin. They headbutt. Doesn't hurt much. They're easy to kill but come in packs, or herds, so several headbutting at once coould kill you if you're low on health to begin with. That'd be one of the most embarrasing deaths, besides the bloatfly.