Varmint is an informal word that means "wild animal" or "critter." Your uncle Bob might see a mouse and yell loudly, "Get out of my kitchen, you varmint!"
You won't be surprised to encounter varmints during a walk in the woods, though you probably don't expect to run into them in your bathroom or at the library. A pesky animal is most likely to be referred to this way, like a raccoon that keeps knocking your trash cans over. You can also call a human a varmint, especially if it's someone who's driving you crazy. Varmint, since the 1500's, has been an American slang version of vermin.