Derpy was a real smart filly. She got good grades, got into higher classes, and she wanted to become a scientist. She was mixing the chemicals very, very, carefully. Then suddenly, Rainbow Dash executed a Sonic Rainboom from outside the school and gained her cutie mark. The sudden blast startled Derpy, and she spilled the chemicals, making a huge explosion. When she woke her eyes were mixed, and she hit her head hard enough for her to forget all he education. Doctor Whooves came into the scene with his Tardis. (sorry if i spelled that wrong) He was frustrated, and he said something about coming in to late, and then he just left. Derpy had no idea what just happend. Then she became a mailmare, and continues to do what she does to this very day. And here is a comic to back up my theory.