I used to think Crookshanks was
Mundungus Fletcher's animagus form, because in the books they are described similarly.
Look in OotP, pg 22. There's a description of Fletcher - second to last paragraph.
Now, if you look in PoA, pg 60, there's a description of Crookshanks.
Very similar, am I right?
Also, Wouldn't the name fit? CROOKshanks? Mundungus is pretty much a crook, right?

But, I keep reading that Rowling says Crookshanks is not an animagus. . .so to heck with it.