>>36145538The bulbous tip of Mewtwo's tail is a second brain while the tail itself is more like an umbilical cord that connects it to a second spinal cord connected to its head through the tube on its neck.
Mega Mewtwo Y has the two nervous systems fuse to augment its psychic power, fusing its "tail" with its head.
I read this somewhere and I'm not sure if it's official or someone's headcanon.
