Tonberry.
Cute and cuddly...and he'll slice you up like Pyramid Head if he gets close to you. Alternatively, there's always "Everyone's Grudge"...
And then there's Wobbufett
Suffice to say that if you see one, you'll kill it, but there's a darn good chance that you'll lose whatever pokemon goes out to face it. While this makes it annoying at the best of times and dangerous if you're in an actual match, seeing one can be absolutely horrifying if you're doing a Nuzlocke run, which dictates that any pokemon that faints must be treated as if it were killed (meaning you release it at the next opportunity).