Ploot

Ploot, A.K.A. Experiment 505, is an illegal genetic experiment created by Jumba Jookiba. He is designed to flood entire cities with a thick toxic sludge created from ordinary trash and pollution he collects, growing larger with all that he accumulates. His one true place is cleaning up litter and pollution at the beach. Ploot is voiced by Frank Welker.

Lilo & Stitch: The Series
Experiment 505 was the 505th genetic experiment created by Jumba with Hämsterviel's funding. He was designed to collect pollution and trash, growing exponentially bigger as he hoarded more. When 505 collected enough, he would then convert the pollution into toxic sludge and spread it across the nearby area.

505 and the other first 624 experiments were deactivated and smuggled to Earth by Jumba during his mission to capture Experiment 626.

All of the experiment pods were released and scattered across the island of Kauai.

505's pod eventually made its way into a bag of trash, which was thrown over a cliff by a couple of polluters into Pudge's underwater grotto, activating 505.

505, named Ploot, first began collecting pollution in the grotto, then he started collecting trash all around town, but not before Lilo and Stitch discovered him while they were busy cleaning up the island.

Lilo and Stitch, unaware of what Ploot was collecting all the pollution for, decided to let him carry out his primary function rather than attempt to stop him – that is, until Jumba revealed Ploot's true intentions.

Eventually, Ploot collected enough trash to grow into a monstrous form, and began his rampage by spreading toxic sludge around the island of Kauai.

While an ailing Stitch was recuperating from an earlier garbage-eating incident, Lilo rallied several other experiments to try and stop Ploot. Despite their best efforts, Ploot was simply too strong for Lilo and Stitch's cousins.

Before long, all of Kokaua Town was covered in Ploot's thick toxic sludge, which contaminated everything, including Pudge's grotto.

Ploot eventually kidnapped Lilo, ignoring her advice to change his ways, but she was saved by Stitch, who had fully recovered from his illness. Lilo then defeated Ploot by using natural air freshener (a household agent that Jumba and Pleakley discovered) to reduce the latter to his original size.

Ploot was then captured, rehabilitated, and found a one true place cleaning up litter and pollution.

Leroy & Stitch
Ploot was seen early in the movie as Lilo was making her rounds with Scrump to make sure all the experiments were happy in the one true place they belonged.

Later, the first 624 experiments, including Ploot, were rounded up by Leroy and taken to a stadium to be destroyed. However, Lilo, Stitch, Jumba, Pleakley, Reuben and Gantu arrived before the experiments could be destroyed.

Ploot participated in the following battle between the experiments and the Leroy clones.

The Leroys soon gained the upper hand in the battle, but were defeated when Lilo, Stitch, Reuben and several other experiments performed the song "Aloha `Oe," which caused the Leroy army to shut down due to the original Leroy's fail-safe.

Stitch! anime
Ploot made an appearance in the Stitch! anime where he grew to his monstrous size and polluted everything again. This also caused a chain reaction since the island was somehow the center of Earth's power, and it nearly destroyed the planet.

Trivia

 * In the episode, Ploot is erroneously referred to as 515, which is a number taken by Deforestator who as well appears in that episode.
 * The episode he was in was aired on Earth Day.
 * Ploot's pod color is blue.
 * Ploot is described by the experiment computer screen as, "Experiment 505 Primary function: Polluter."
 * In Leroy & Stitch, Ploot was seen in his monster form during the battle between the Leroy army and the experiments.