Qwertier

Apparently my title's too long again

The reason why neither Fred and George not Harry ever saw Peter Pettigrews name on the Marauders Map:

(By the way this from tumblr. I am in no way clever enough to have thought of this.)

Ever since some one on tumblr pointed out that it doesn't make any sense that Peter lives with Ron for three years and yet Fred and George fail to notice this on the marauders map, I've been wondering how this can be explained. I finally figured it out!
It's because the map shows the names of animagi only when they're in human form, and in animal form, only if the maps user knows that they are animagi. Think about it, nobody except Lupin, who had witnessed their transformations countless times, was able to see Peter and Sirius in animal form on the map. Snape only saw Lupin and Sirius in the shrieking shack (who were both in human form) but not Peter (who was a rat at the time). Harry also never saw Rita Skeeter on the map when she was a beetle.
I'm pretty sure the marauders included this feature because the should the map ever be found by a teacher, their being animagi would still remain a secret.  This would've been more important to them than the map being completely accurate. Lupin didn't even tell Dumbledore that Sirius could turn into an animal when he thought Harry's life was in danger. That's how much he wanted to keep it a secret, even over twelve years later.