Scrappy Rex was a giant monster Scrappy-Doo morphed into after absorbing all the protoplasms.
In this form, Scrappy grew to immense size and also his fur tone became slightly darker. His facial features became more monstrous and gained rather large sharp claws and teeth. His eye color also went from brown to glowing green. All of this was sustained by keeping the Daemon Ritus attached to his chest.
Fitting his huge ego, Scrappy was still obnoxious and often ranting on how powerless Scooby and the others were. He became a power hungry monster who wanted nothing more to obtain power and destroy Mystery Inc. Scrappy showed extreme hatred for Mystery Inc., especially his uncle. Thus their relationship may have become extremely strained.
Powers and abilities
Due to absorbing and consumption of many protoplasms, Scrappy Rex had immense strength and control over the demons.
Scooby-Doo: The Movie
He only needed his uncle's protoplasmic soul to complete his transformation. He then chased Shaggy Rogers and Scooby until he had them cornered. He then managed to catch Scooby after a demon possessed Mary Jane, Shaggy's girl friend, handed him over. Scooby told Scrappy to sit down and then hit his nose. Scrappy roared angrily at Scooby. Scrappy handed Scooby over to the pincer to take out Scooby's protoplasm. But then the Mexican wrestler, Zarkos, after his defeat by Daphne Blake, fell over the vat, releasing the people's protoplasms. Scrappy was angered by this, telling the Mystery Inc. gang that it wasn't over. Shaggy then defeated Scrappy Rex by pulling the Daemon Ritus out of his chest and destroying it. Scrappy Rex lost all of his protoplasms and morphed back to Scrappy-Doo.