Derpy became an alicorn when Luna invited her over for lunch. There was a wild berry muffin that Derpy ate. Then she stuffed a normal muffin in her mouth. Then with swirling lights and every thing she became QUEEN DERPY!!!!!!!!!!!!!! Princess Luna became her servant along with Celestia. In this picture you can see Celestia thinking why I am a servant to this muffin-eating-crazy-alicorn! ( Derpy is her huge bed)
Thank you @Degong
