The Giant Spongey Monster is a one-shot My Life as a Teenage Robot villain who only appeared in the episode "Speak No Evil".
While on a trip to Japan, it was given to Tuck as a gift. Tuck ignored the instructions that stated not to water it and it sprouted into a large sponge-like creature. The Spongey Monster was defeated by Jenny when she rung the creature dry, reverting it to his original state. The sponge was most likely thrown away to prevent Tuck from doing the same thing again.