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 and 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 the 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 fights Eustace the Kangaroo Monster. The only thing that can defeat a kangaroo monster is another kangaroo monster.
Eustace throws a stone tunnel at Courage, but it hits Eustace after Courage blocks it. Courage throws a glassed pyramid-like design at Eustace, but Eustace destroys it. Eustace throws gargoyles at Courage. Then the 2 giant monsters both paint pictures to see who's best. Eustace painted a stick person, but Courage painted Mona Lisa.
Eustace climbs on the Eiffel Tower and Courage climbs to stop Eustace, but a french police officer told them that the Eiffel Tower allows only 1 kangaroo monster, but not 2, so Courage gets off the tower and Eustace laughs at him.
At the other landmark, 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 took the kangaroo monster bone out of Courage and stated 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.