The buttocks fight is a coordination game and fighting game for always two competing players. Of course, you can easily create some kind of tournament to allow more players to participate and determine a winner.
You don't need any equipment.
The players are standing on a small playing field (you can mark it with mats, ropes, chalk, etc.) They are standing back on back (or better: buttocks on buttocks). Once the game starts, both try to bring the other player out of balance, so he topples over or has to step away. The first player that moves away from his standing position (even a small step) loses the round. Usually, three points have to be scored to win the game.