Ms Norris has gotta be a half kneazle. Her smarts and her communication skills are so like Crookshank's (who def. is a half kneazle- Jo said) that I think, Mrs Norris has to be one too.

Kneazles are magical creatures known for their intelligence and for being highly perceptive animals. They look like large cats. Mrs Figg breeds half cats/ Kneazles.

There's been a few discussion threads on kneazles on this site if you want to find them through your search engine or just google 'kneazle' for Jo's explanation of them.

That's my first theory.

Second theory:
Mrs Norris is demon manifested from the malice and spite of Filch. Kinda like a poltigeist is created by teenage frustration, demon cats can 'come' to annoying little squibs.
Whatya think??