Spooky

 Spooky, A.K.A. Experiment 300, is an illegal genetic experiment created by Jumba Jookiba. He is designed to morph into his victims' worst fears. His one true place is haunting the old, abandoned mansion, and (on Halloween) scaring trick-or-treaters. Spooky is voiced by Frank Welker.

Bio
Experiment 300 was the 300th genetic experiment created by Jumba with Hämsterviel's funding. He was designed to terrify Jumba's enemies into submission by morphing into their worst fears.

300 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, with 300's pod landing on a house roof.

Lilo & Stitch: The Series
On Halloween night, a gust of wind caused 300's pod to roll down the roof's gutter and into a barrel of water, activating the experiment. 300 then noticed a nearby mouse, and scared it off by turning into a wild cat. When a dog attacked 300 because of the cat form, 300 scared away the dog by turning into a dogcatcher, then fled the scene in his elastic form.

300 shortly after attacked a children's Halloween party, scaring everyone off by turning into a headless Lilo, and then a giant spider.

Later, when Lilo and Stitch arrived and Stitch entered the kitchen to raid the fridge, 300 locked himself in the kitchen with Stitch, and almost drowned the latter by turning into floodwaters. However, 300 was forced to retreat through an air vent when Lilo noticed his presence.

Sometime later, 300 arrived at the Pelekais' house, where he terrified Nani by turning into Cobra Bubbles and claiming he came to take Lilo away. However, when Lilo and Stitch returned to the home and noticed Cobra to be 300 from his hissing voice, they exposed 300, named Spooky, as an imposter, forcing him to retreat.

Lilo, Stitch, Jumba and Pleakley followed Spooky into the old, abandoned mansion. Spooky startled Stitch, causing him to fall into an indoor swimming pool. He then surprised and terrorized Lilo by turning into a clown, then defeated Jumba and Pleakley by turning into their worst fears: Pleakley's mom and Jumba's ex-wife.

Stitch started sinking to the bottom of the pool while Spooky chased after Lilo in his clown form, scaring her to the point where she hung on a ledge for dear life. However, Stitch soon overcame his fear of water by singing Aloha `Oe (a song Lilo taught him as a coping method), and floated to the surface. He then rescued Lilo.

When Spooky noticed that Stitch had faced his fears, and was thus not affected by any of Spooky's forms, the former turned into a bat and attempted to escape up the chimney, but was trapped by Stitch in a containment orb.

Spooky was then found a one true place scaring trick-or-treaters on Halloween, and (on other occasions) haunting the abandoned house.

In "The Asteroid," Spooky was one of the six experiments who Stitch convinced to come with him and Lilo to save Earth from an approaching asteroid. During the journey, Kixx kicked the back of Spooky's seat, causing the latter (using his ability to morph into nightmarish forms) to retaliate. Spooky later attempted to use a monstrous form to scare Pleakley into giving him free tea, but Pleakley only replied with an order for Spooky to return to his seat.

Spooky was one of the experiments in "Checkers" who joined Stitch in a rebellion against Gantu.

Leroy & Stitch
The first 624 experiments, including Spooky, were round 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.

Spooky participated in the following battle between the experiments and the Leroy clones by using nightmarish forms to scare Leroys.

However, 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 failsafe.

Stitch! anime
Spooky made an appearance in the Stitch! anime when Hämsterviel sent him along with Toons to scare the children of a summer camp. It is shown that he uses a form of telepathy to read his victim's mind and discover their fears.

Trivia

 * Spooky greatly resembles the Disney (and Halloween based) character Oogie Boogie.
 * Spooky's pod color is green.