Cow legs on the back is one of my favourite yoga poses to stretch my glutes and hips. Doing the pose on your back instead of seated can be more accessible for some people.
How to do the pose:
Lie on your back and bring your knees towards your chest. Cross your right knee over your left as if you are sitting cross-legged in a chair. Reach for your ankles and bring them in towards your hips. For more of a stretch you can bring your ankles out to the side as seen in the second photo. As you hold the pose intend your right hip forward towards the front of your mat for more sensation. Hold for as long as you like and then repeat with left leg on top.