Mrs. Norris is Argus Filch's cat. She doesn't seem like she's capable of speech, but she's clearly got some way of communicating with Filch. When Harry goes up to the Owlery to send a letter to Sirius and canoodle with Cho, Mrs. Norris spots Harry and runs to find Filch. Filch comes storming into the Owlery accusing Harry of ordering Dungbombs against school rules. This is completely not true, and later, Hermione puts together that someone (a.k.a. Professor Umbridge) tipped Filch off in the hopes that he would search Harry's mail. However, the important point is that Harry believes so firmly that Mrs. Norris can communicate with Filch that he shouts after her (the cat, mind you), "I'm not doing anything wrong" (14.10). He thinks she has the "unmistakable air of a cat that was off to report to her boss" (14.11).