As of the first three chapters of The Prisoner of Azkaban, we know the following:
- Aunt Marge breeds Bulldogs. As she's in England and they're described as fairly big, they're probably English Bulldogs. Furthermore, as she's a professional breeder, they're probably fairly normal dogs. In other words, they're probably not mutants with massive tails.
- This year, she brought a specific Bulldog with her. She lets it drink tea, further suggesting that they're English Bulldogs.
- Last year, she brought an unspecified dog with her, and Harry accidentally stood on its tail.
If the unspecified dog wasn't a Bulldog, or was a strange breed of one, then I have no problems with this. However, the context strongly suggests that it's a Bulldog and out of all of the Bulldogs that I've known, modern English or modern French, I can't believe that it's possible to step on their tails. Wizards do have some funny things happen around them, but I can't devise any way to step on a Bulldog's tail without having to step on the entire backside of the dog. In other words, if you were going to accidentally step on it, you'd trip over the dog.
This gives me my question - what do we know about the dog that Harry stepped on the tail of? If it's a typical English Bulldog, then I don't see how it's possible.