For some time now, I've been wanting to draw a Rainbow Dash pinup - first, because she's a beautiful character to draw and color, and second because I think it's pretty hard to do Rainbow justice while also making her tempting.
I think this is because I believe good sexy art doesn't break character. I don't always follow this rule myself, mind you, but ideally your image will be much more effective when you can really believe that pony's reaction or situation. Dash isn't exactly too likely to lounge around sexily for your benefit - unlike, say, Rarity. She can hardly sit still. As a result, Dash has to be unexpectedly sexy - probably not even aware of it herself.
So I went with the age old "gawking at sexy athletic girls" trope, the post workout winding down. Really, this could've been in the locker room at Cloudsdale High, but she's all alone and just for you! Butters had suggested stretches, but if Bon Bon is any indication, I think pony stretches are more cute than sexy. And, of course, there's fewer dangerous curves for those stretches to highlight.
Also, according to my research, Rainbow is at least 60% sexier when she's annoyed. So in that goes too.
I'm very pleased. Coloring this, pressing my nose just centimeters away from her rump while stroking in all that blue ink - it's enough to lead a girl to distraction.
Marker and colored pencil atop watercolor wash with white acrylic highlights, 2012.