It's a Dangerous Business, Going Out Your Door/WMG
The World Serpent is the predicted disaster the three main characters will unleash.
They might have bested it, and gotten away with the flower, but the World Serpent is still alive and very angry. So it's going to come down off the mountains and flatten the Gazelle living below... and then move onto Gildedale. Applejack, Rainbow and Rarity saved the Gildedales from constant battles only to unleash the thing which is going to take an army to stop if it indeed can be.