Hrm... maybe the grid just needs a bigger bum. I had a sloppy drawn artistic masterpiece bow hidden in one of the fields, but I'm going to reward crayons to anyone who hit one of the fields that can still qualify as buttocks as soon as the bow is found.
It is B5!! Derpy manages to tie a wonderful bow around the mysterious guest's tail. Meet the world's most fashionable elephant:
Additional buttocks fields are A5 (unclaimedBiomecha too, but no need to spank the elephant twice), A6 (Biomecha), A7 (mdom), A8 (unclaimed) and B8 (unclaimed)