In the Sorcerer's Stone, it says first years are allowed to bring an owl, cat, or toad. Then why is Ron allowed to bring his rat?

Joahn Rowling said in a very recent interview to some fans that asked her ,the same question ,with you and said that Rats are allowed in the school coz of a very ancient tradition in Howarts.Hope I helped you!

Thats a good question because in POA Ron said the rat was Percy's first.

Maybe the owl, cat, and toad are the common ones and they don`t name the unusual ones.