The Grim Adventures of Billy and Mandy (2001)
S3E16 Which Came First?
Episode Overview
On a trip to a science museum, Billy and Mandy's school bus malfunctions in the desolate desert. While searching for fossils, the trio of Billy, Mandy, and Grim stumble upon an old, deserted mine, where Billy is snatched by a gigantic, mutated chicken. As Mandy and Grim race to save him, they discover a Cold War-era lab where these bizarre chickens were engineered. Just then, the mountain erupts, sending lava that cooks the chickens. Thankfully, the entire class ends up with a feast, avoiding any dire need for cannibalism.
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S3