They were a give away around Valentines day I think. You could buy them for an amount of Farm Cash in the market. Currently, they can be placed in the stables like the horses and produce foals when a wandering stallion is in the stable with them. Unless you have a pink pony to breed from, you'll only be able to collect pink pony foals from any of your neighbours who may release them onto the feeds from their own stables. Hope this helps.