Wait, I just checked and there is no grass in the Islex forest (still wild pokemon). You did encounter it only in the forest? I once heard of a missingno-like thing that appeared on Lugia's islands. It was described like an Unown with a squiggle on the bottom going all the way off screen. The poor kid warned me of it after showing me his Celebi he got from a trade or something. Then it came back and erased his file again.