Derpy explained

So, I was talking about Derpy, when something came up. Derpy was a full grown mare when Rarity got her cutie mark, a child when Rarity lost that prize in the race, and a child when Rainbow Dash was little. We also have the doubles. My theory is that Derpy is a clone. Her derped eyes are a side effect of being cloned so much. Or there is the possibility she is a time traveler (Dr. Whooves). I don't know, maybe she is derped up because of time travel...