Megabug

The Megabug is the true main antagonist of Mario + Rabbids Kingdom Battle. It initially manifests as a blue vortex through which the Rabbids are transported to the Mushroom Kingdom, but by the end of the game, it assumes a physical form, appearing as an orange dragon-like creature.

At the end of the game, it merges with Bowser, creating MegaDragonBowser. Mario and his friends are able to defeat it and separate it from Bowser and destroy it.