How it kills: Particularly a problem in the Sunshine State and the bayous of Louisiana, alligators can crush human bones—meaning your arms and legs—with a single bite. Like other wild animals, if you feed them, they'll associate food with humans, potentially turning you into dinner in your own dining room.
How to get it before it gets you: These guys are big, so there's not much chance of them sneaking into your place through a crack or hole. But leave a door open and they'll mosey on in, and they like a waterside backyard as much as you do. If they're abundant in your area, consider installing a wire mesh fence around your backyard, especially if you have a pond or pool. And remember: Don't feed them.