Scabbers

Ron had no proof that Hermione's cat Crookshanks ate his pet rat Scabbers, yet even if he did have concrete evidence, it's not like Hermione has any control over the actions of her pet.

Sirius has escaped from Azkaban, Peter has always been there disguised as Scabbers, Lupin is a professor, and even James returns in the form of Harry's majestic stag Patronus.

Ron is angry because he feels Crookshanks is responsible for Scabbers' disappearance, while Hermione fiercely maintains that Crookshanks is innocent.