The best cardio machine to develop and shape your butt is a machine that requires you to move your thighs and hips, and there are a number of machines at your local gym or retailer that will do just that. The muscles in your butt are called your glutes, and the machines to use for developing and shaping these muscles are the ones that cause you to move your thighs backwards and straight out from your hips.
Walls and Ladders
A climb mill or treadwall simulates the actions of rock climbing, which involves the range of motion in your thighs and hips that engages your glutes. Variations on this cardio machine include the climber, which is a machine that simulates the motion of climbing a ladder, and the ladder mill, which sits at an angle, requiring you to move just like a dog would do climbing stairs. The added benefit to all three machines is that your knees and shoulders get a workout too.
Cross-training For Success
An elliptical cross trainer machine works several muscles at once, including your hips and thighs. The machine is a mixture of running and climbing stairs, with the added bonus of engaging your upper body. If you're looking for something a little more gentle than a climb mill or treadwall, this is the machine for you. Variations of this machine include the elliptical glider, elliptical trainer, and seated elliptical cross trainer. Like the treadwall and climb mill, elliptical trainers offer the benefits of a weight-bearing workout, helping you to build strong bones.
Hitting the Virtual Slopes
Skiing requires you to move your thighs and engage the muscles in your butt, and a ski machine provides the same tough cardio workout without hitting the slopes. The machine comes with or without rails for you to hold onto, with the former offering the greater challenge. As for the difficulty level, it falls somewhere in between the climb mill and elliptical cross trainer. It won't give you the full-body workout that the other two types of machines provide, but your thighs and butt will almost exclusively be targeted.
Climbing Stairs and Skating
The stair climber and stepmill are cardio machines that simulate the action of climbing stairs, and both of these machines will develop your butt. The exercise can be tough, but you can adjust the machines for speed. If you like using ski machines, but are in the mood for something a little more gentle, a skate machine simulates the motion of skating on ice. It's a gentle cardio workout, but your core, thighs, hips and butt are all engaged.