Discord wishes to experience life as pony for a day, so a spell to temporarily separate the parts he is made up of is cast, but it backfires due to his inherent chaos powers, turning everyone in Ponyville into an amalgam of other ponies. Even worse, separating his “draco” from his “equus” results in a Discord-pony who’s completely powerless, and a dragon-like creature who’s back to Discord’s old vengeful self! Discord-pony must team up with the amalgamated Shylight Sparkle, Rainbow Jack, Pinkity and Princess Lunalestia to turn everything back to normal.
Actually had a similar idea where the CMC get melded together into one body (so they’re technically an alicorn), but only one head can be on the body at a time (they try having two of their heads out at a time, but it’s like having your cheek pressed up against your friend’s cheek; kinda awkward). Legs, wings, etc, are controlled by the Crusader they belong to, and can be swapped in and out at any time. Each part can be controlled at any time, not just when that Crusader’s head is out. And they can all communicate with each other, somehow.
This will lead to weird shenanigans with Sweetie Belle telling the others they need to get help, so she tries to move forward with her front legs, but Apple Bloom doesn’t want to be hovered over by Applejack again, so she digs in her hind legs, and Scootaloo is still in panic mode, so her wings are flapping all over the place, slapping Sweetie Belle in the face.