Kangaroo Monsters are a species which were seen in "The Transplant".
According to the Computer:
"The Kangaroo Monster is a very rare, very extinct, very huge creature. A social outgoing creature that loved to crush anything that moved to a pulpy pancake. But that was then, this is now. They're all extinct and the bones are very, very valuable."
Courage finds a bone. This attracts archaeologists who end up uncovering a huge fossilized dinosaur-like kangaroo skeleton.Courage researched the bone on The Computer who in turn tells him the bone Courage found came from a prehistoric monster called the kangaroo monster.
While selling bones to bidding archaeologists Eustace fell off the roof and one of his side bones was damaged. He was revived with a kangaroo monster bone was transplanted into him, but he turned into a real kangaroo monster and captured Muriel and went on a rampage. Eustace destroys the Empire State building in New York, destroys Pyramids in Egypt, and destroys the Great Wall Of China . Courage told Dr. Vindaloo that the kangaroo monster bone has turned Eustace into a giant kangaroo monster. To help Courage combat the mutated Eustace Dr. Vindaloo transplants another bone in Courage to turn him into a kangaroo monster. Courage then goes to Paris to fight Eustace the Kangaroo Monster, as the only thing that can defeat a kangaroo monster is another kangaroo monster.
Eustace throws the Arc de Triomphe at Courage, but it hits Eustace after Courage blocks it. Courage throws the glass pyramid that stands in front of the Louvre at Eustace, who catches it in his mouth and crushes it. Eustace throws gargoyles from Notre Dame at Courage, who ducks. Then the 2 giant monsters both paint pictures to see who's best. Eustace paints a stick person, but Courage paints the Mona Lisa.
Eustace climbs on the Eiffel Tower and Courage follows to stop him. A French police officer arrives and tells them that the Eiffel Tower allows only 1 kangaroo monster, so Courage gets off the tower and Eustace laughs at him.
Across from the Eiffel Tower, on the other side of the Seine, Courage pulls out a slingshot from his pouch, loads it with a croissant and shoots it at Eustace. Eustace simply eats the croissant. Courage shoots off 2 more croissants, and Eustace eats them as well. Courage then begins shooting a multitude of croissants, which quickly engorge Eustace, causing him to become so massive he begins to bend the Eiffel Tower which he is hanging off of. Courage fires off 3 more croissants, which increase Eustace's weight to the point that that the tower cannot support him anymore, and he is dropped into the water, allowing Courage to rescue Muriel.
Dr. Vindaloo removes the kangaroo monster bone from Courage and states that the kangaroo monster bone will have no side effects when taken out. Courage happily hops along with Muriel who walks.
Back at Paris in a building with a stadium, the French Officer puts a gold medallion on Eustace and declares him the "genius Kangaroo Monster", awarding him for his appreciation of French pastries. Eustace wins a massive pile of pastries, much to his delight.