Mayor Bloomberg’s pregnant daughter, Georgina, has picked a pony named Harley for her unborn son. “I have one little rescued mini that I have already singled out,” the equestrian told us Saturday at the Ferrari tent at Bridgehampton Polo. “We’re gonna try popping a little saddle on him, and get him ready. We have a couple of years to get him all trained up. I kind of always said when I first rescued this little mini that I wanted him to be my first child’s pony. It’s perfect timing.” She’s due Dec. 24.