Trevor The Toad --

Actually, contrary to this popular belief, owls are actually quite dumb.

How so?

We're talking magic owls, here.
We accept that they are smart because that's the premise.

I reckon animal forms of animagi do show up on the map because remember at the end of book 3 Lupin said that Harry, Ron and Hermione left Hagrid's cabin with someone else - Peter Pettigrew, but he was still a rat then. I guess the main reason why Harry didn't notice PP on the map in the book (in the movie he does) is that he wasn't looking for him. he would normally find himself on the map and then check no one who could get him into trouble was en route to him - as long as PP was aways from him he wouldn't have mattered.

As far as Neville's Uncle is concerned - I don't have book one here so I can't look up his name - who says he's a good guy? We assume Neville's grandma is... but not everyone in a family is the same way.

I just remembered. Crookshanks would notice.
He recognized Peter and Sirius right away, not just as animagus, but as good or evil.
He tried to attack Peter countless times, but he helped Sirius and kept his secret.
Also, Ron believed Crookshanks could be trusted to spot Pigwidgeon if he was a bad animagus.
Can we safely assume that the Crookshanks test is valid, and that "Pig" is clean?

I agree Crookshanks would notice (I wonder if Hermione ever thought to have him in the same room as Mundungus Fletcher, what would he make of him?, but I digress). Plus, with Lupin watching the map and seeing the Animagus Peter there we've got a fairly sure fire way of discovering Animagi.

Yep, and I think that we can trust Crookshanks instincts about Pigwidgeon.

I agree. We know that Crookshanks is half Kneazle. In beasts we're told that kneazles are intelligent, independent and have an uncanny ability to detect unsavoury or suspicious characters. Just like Crookshanks detected Scabbers for what he was and Padfood for what he was as well.

I wonder want he would do around Snape?

Crookshanks and Snape have been in the same room - in the Shrieking Shack at the end of book 3. I don't think Crookshanks did anything.
But hey, do we know what Crookshanks might do around Mundungus Fletcher? I mean, he's sort of been the crook of the last couple of books.