At the beginning of your upkeep, return a creature you control to its owner's hand.
7/1/2012 Roaring Primadox’s ability is mandatory. When it resolves, if Roaring Primadox is the only creature you control, you must return it to its owner’s hand.
7/18/2014 The ability doesn’t target any creature. You choose which one to return when the ability resolves. No player can respond to this choice once the ability starts resolving.