In the Harry Potter films, the Hogwarts caretaker is a Squib named Argus Filch. He can usually be seen holding his pet cat named Mrs. Norris. According to the Harry Potter Wiki, Mrs. Norris has "an unusually strong connection with her master, alerting him to any students misbehaving inside the school grounds."
I understand that there are cats that are smart and intelligent and loyal to their masters but if it is to the point where it is unusual, then that's saying something. I can be skeptic, but I am going to tell you a theory about how Mrs. Norris is actually an unregistered Animagus, and could even be trapped like this.
First off, Filch can't be married to a cat. But the two have almost a romantic relationship. As we can see in Goblet of Fire, Filch and Mrs. Norris are dancing together.
Another piece of evidence that might mean that Mrs. Norris is an Animagus is that she appears on the Marauders Map, according to the Harry Potter Wiki.
In the film adaptation of Prisoner of Azkaban, Harry looks at the Marauder's Map and sees Peter Pettigrew. However, he is in Animagus form (a rat), so Harry can't see him. But he appeared on the map despite being in the form of a rat.
The Marauder's Map is not fooled by Animaguses.
And now you could be saying that all animals appear on the Marauder's Map and Mrs. Norris is an animal. Well, when we look at the Marauder's Map, we never see anything like "Spider" or "Ant". And if Fred and George used that map during the time the Chamber of Secrets was opened, then that means the basilisk would have been discovered a lot sooner.
Well, that's all I have for now. If you have any other possible reasons Mrs. Norris may be an Animagus, comment down below! :)