Mountain Lions (puma, panther, cougar, whatever you want to call them)
When hiking through the woods nothing is scarier than hearing the scream of this big cat close by.
Running may not be your best option here. Like a house-cat, they enjoy the thrill of the hunt and will become more intrigued when their prey tries to escape. Instead, act aggressively, usually the cat will back down but if not, get ready for a fight.