Rat Bikes! Pub Bikes Beaters! Frankenbikes!

just because it's dirty doesn't mean it's a rat
it just means it's been ridden ... regularly

there should be rust / scratches / a broken spoke or two and some truly horrendous parts on a rat

the last two examples shouldn't be here surely

depends on whether you consider the term "rat" to be pejorative.

To me that serotta is a rat. i.e. a decent quality bike showing very obvious signs of heavy usage.
Beaters on the other hand are anti-theft, low cost, low-maintenance.

To my mind a rat doesn't need horrendous parts or poor maintenance - rats are ridden with pride.