I think it is easier to guess who it is if you eliminate the obvious.

It can't be anyone considered pure blooded -- so that leaves out the Weasleys as well as Dumbledor (unless he is half blood)
I also assume that it must be a wizard -- so that leaves out crookshanks

I agree with an earlier guess -- it is probly someone we have seen a glimpse of in an earlier book -- but was not a prominant character
(like when JKR said she was killing an important charactor in GoF and it turned out to be Cedrick -- I had not noticed him much before that book.)