Board Thread:Wikia Questions and Answers/@comment-69.11.40.130-20141214182000/@comment-3474542-20141217091138

You can store most of them in the Baby Playpen or the Nursery Barn. If you store them in the Nursery Barn, there is a chance they will transform into an adult animal (not necessarily of the same kind as you put in) when you harvest the building. If you want a guaranteed adult version of the baby foal/calf/fawn, you need to click them and use the "Grow Up! (Farm Cash)" menu dialogue (this only works for certain animals).