"I like big butts ..."
Okay, giggle now, but know quarter and paint horse (well, and Gypsy Vanner, draft -- heck, a whole bunch of horse people) like big butts, too!
The high quarters of a horse are her engine — the power behind the heart.
I chose black and white for this image because it removes the distractions of color in the background. The mare in this image is a blue roan, so the main subject was black and white anyway, which meant the greens, browns, blacks and auburns of the background drew the eye away from the focus.
Other than desaturating the color, this image had very little post process in Lightroom.
As to why I took it -- well, when you standing with a camera in a herd of horses for 2 hours, you start to look for any new angle. (smile)