Awww! Yog-Sothoth is adorable!
I'm not sure which kind of creature came into Timmy's neighborhood and affected him, but I think it could have been similar to one of these or some related ... um, is species appropriate? ... and I'm not reading Lovecraft to become more expert in the matter.
No.
The being was literate, we know that, although no one including Timmy has any idea what it looked like. At least it's gone.
Maybe.
I think Shub-Niggurath is cuter.