The Reptilian Clone appears in "We Fixed a Truck" and replaces Princess Bubblegum for an unknown reason. While the lizard is masquerading as Princess Bubblegum, BMO offers it a bug, which it quickly grabs with its tongue and eats. It then transforms into its true form and starts attacking Princess Bubblegum's castle. Banana Man drives Hot Daniel up a ramp, launching the truck into the lizard's mouth and killing it.
It appears the final boss in Royal Ruckus, where it is called Replicant.
They think I've been replaced with an evil lizard replicate. It's sheer nonsense!