Saw a beached box jellyfish once, when I was about 12. Instantly killed whatever interest I still had in beaches, once I found out how fucking dangerous they are, and made the oceans about 100 times more scary in my child-sized mind.
Also had bears rummage through my campsites, on two separate nights (in different locations) in ONE week-long camping/horseback riding trip.
And, while not exactly the most dangerous of critters, there's a family of about 5 or 6 raccoons living in/around my back yard. Don't laugh. Not only do they carry rabies, but from what I hear, when they attack, they go right for your eyes.
Oh, and there's this buck (male deer) who comes to my front lawn every night and just stares down anybody who comes by our house. One time I made the mistake of coming home late and had the scariest time of my life trying to sneak into my own god-damned house without getting run down.