Ron Weasley has his pitiful pet rat Scabbers with him from almost the get-go. This rat is brought with Ron to Hogwarts for multiple years, and presumably Percy brought Scabbers to Hogwarts when he owned the rat.
Now the papers that Harry received with his acceptance letter clearly stated that a student may bring with them an owl OR a cat OR a toad. Nowhere is it mentioned that, should they be unable to afford an owl, a cat or a toad, then they may bring a rat. But no one at Hogwarts seemed to take notice or mention this fact.
So why was it that Ron (and presumably Percy) were permitted to bring a rat to Hogwarts?