I was rereading OOTP last night, and I noticed something that completely squashes the the possibility that Crookshanks is Mundungus. The night that Harry is brought to headquarters they are all sitting eating dinner. Mundungus is there, talking to Fred and George. And Ginny is playing with Crookshanks on the floor.
Yes, my theory did not hold water. BUT I think there is something that JKR is trying to clue us in on about the cats. Why would Mundungus look so much like Crookshanks? I have a lot of suspicions about the cats. Are they all cats or are they transformed wizards in hiding?
Well, we KNOW from JKR herself, that Crookshanks is not an ordinary cat.
She tells us to look for clues in one of her books - Fantastic Beasts & Where to Find Them
:::>^..^<::: ~*~The Journey is more important than the end or the start~*~ :::>^..^<:::