Sea monkeys

Trying to impress his friends, Fry buys some sea monkeys from Shifty's True Wonders of the 20th Century Emporium. After pouring his new pets into a bowl he discovers that, after a thousand years, the sea monkeys have all died. Instead of flushing them down the toilet, Professor Farnsworth has Fry to 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