The survivors of the changeling swarm that invaded Canterlot have learned their lesson: ponies are too resistant to be taken as a food source through violence. Fortunately, there are other ways for a changeling to feed. It is so much easier just to pick vulnerable ponies and brainwash them into willing servitude. They can even be sent out to bring back more victims!

On a related note, Vinyl Scratch picked the wrong night to leave the club with a pair of unfamiliar ponies.