Sea monkeys

Trying to impress his friends, once bought some sea monkeys from Shifty's True Wonders of the 20th Century Emporium. After pouring his new pets into a bowl he discovered that, after a thousand years, the sea monkeys had all died. Instead of flushing them down the toilet, had Fry dispose of them in a vat of Gamma Radiation. As a result, their tissue and blood cells were mutated and the former pets had an explosive growth spurt, turning them into giants who went on a rampage and started eating New New York.

Trivia

 * The original package of sea monkeys cost $1.
 * The bill for carcase removal was $80,000.