Butt attacks are not to be confused with hip attacks - hip attacks are made with the side of the hip. Butt attacks can look very differently depending on the attack. For example, R. Mika lunges herself, butt first, through the air like a projectile, and rubs her sore flesh if she misses and lands on the ground. Sophitia from Soulcalibur II on the other hand simply places her opponent behind her and then tackles him or her with her butt.
Currently, Princess Peach has the highest number of butt attacks of any character in any game since she has no less than five of them: Peach Bomber, back air, Down Throw, Back Throw and ledge get-in attack. For the record, her Back Throw might look like a hip attack, but it's actually done with her butt (this can be proved by manipulating the camera).