Ever met the kind of person who you don't really like at all, but who has the annoying habit of constantly appearing almost everywhere you go? That's a Rattata. The Rattata is the person who you can never seem to get rid of short of taking out a restraining order (and even then they'll find a way around it). They don't seem to get the message that they're not wanted around.
Named after the Pokémon Rattata, which is one of the most common (and annoying) Pokémon encountered in the wild.
Hey Richard, stop being a Rattata and piss off.
That Simon is such a Rattata, I wish he would just leave me alone.