Monstrosi-Dee Dee is the second part of the 3rd episode of Season 4 in Dexter's Laboratory which aired on May 2, 2003. In this episode, Dee Dee consumes one of Dexter's apples that is used for his research project, and she tells Dexter to hide. Dexter follows her advice, unaware of the fact that Dee Dee is actually the monster.
The title of the episode is a pun on the word "monstrosity" and Dee Dee.
It is assumed that the chemical effects of the contaminated apples eventually wore off on Dee Dee as the episode doesn't seem to show Dee Dee after Dexter panics and runs to warn his parents.
Mom & Dad eat the apples, yet the fate of their consumption was never followed up after this episode.
In the last scene of the episode, an old picture of the family can be seen behind Mom & Dad... Dee Dee is shown with only one pigtail instead of 2... this may be an error, or purposely imposed as logic to capture the early youth of Dee Dee and Dexter.
Dexter don't know that apples were contaminated.
The episode is an homage to the 1955 Bugs Bunny cartoon Hyde and Hare.