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.
They think I've been replaced with an evil lizard replicate. It's sheer nonsense!