Both Sour Apple and Dream Wings have different reasons for needing to go and stay with ponies in Ponyville.
Sour has to go and spend time with his cousin Apple Jack and her friends despite his instant dislike for every pony around him. Dream Wings on the other hand needs to see a very old friend from Canterlot in order to hide from a creature that she has hidden in fear from every day since she was a young filly. However will these two new ponies be the one thing that manages to split apart the Elements and harmony, and consequentially the kingdom, forever? Or will they be the one thing that can stop the nightmare during daytime?