Loaded my farm tonight and got a free Dream Giraffe! I’m not positive of the criteria to earn one, but I’m pretty sure you have to play Dream Zoo by Zynga (currently only for iphone/ipad/ipod touch devices). Not sure yet if there is a certain level that you need to hit, etc.
If you don’t have and i device, ask someone you know to let you play on theirs! The app is free!
Giraffe does go into a zoo habitat, but is an EXCLUSIVE common, which means you have to have one in order to get one.