Here goes:
(_!_) a regular butt
(__!__) a fat butt
(!) a tight butt
(_._) a flat butt
(_^_) a bubble butt
(_*_) a sore butt
(_!__) a lop-sided butt
{_!_} a swishy butt
(_o_) a butt that's been around
(_O_) a butt that's really been around
(_x_) kiss my butt
(_X_) leave my butt alone
(_zzz_) a tired butt
(_o^o_) a wise butt
(_13_) an unlucky butt