I'm a little surprised no one's mentioned this from the Pokemon games:
Zubat and Golbat.
Now, hear me out.
First off, it could confuse you and poison you. Second, in the second generation of games (Gold, Silver and Crystal), it learned Mean Look, so you couldn't swap out or run away to get out of the confusion effects, so you just had to wait it out. Combine that with the poison, and you're easily losing about an eighth of your health a turn. You walk through a cave, they're EVERYWHERE, you walk outside at night and they're EVERYWHERE, and there was no escape. You could try using Repel, but it's a complete waste of money that repels the good Pokemon, too. How can I catch that awesome Machoke [this user is using Machoke as an example, this user actually hates Machoke] when I can't escape these freaking bats?
*sigh*
Rant done.