Dee Dee's Tail is the second part of the 22nd episode in season 2 of Dexter's Laboratory, which aired on January 14, 1998. In this episode, Dee Dee wishes that she were a pony after playing pretend Ponypuff Princess with her friends Mee Mee and Lee Lee, but soon gets exploited by her friends and Dexter.
Dee Dee, Lee Lee and Mee Mee play with the Ponypuff dolls and it's Ponypuff Paradise, while Mee Mee and Lee Lee go for some cookies from Mee Mee's mother, Dee Dee wishes to be a real Ponypuff Princess, the more she thinks about, the more she gets sad about it. She decides she is through playing and barges into Dexter's Lab crying and demands to be a real pony. Dexter rejects at first but then thinks he could use more information on horses. So he agrees, and transforms Dee Dee into a real pony. She then returns to Mee Mee's house, and Lee Lee and Mee Mee decides to ride on her.
After a while, Dee Dee gets worn out from being rode on and cries, Dexter, Mee Mee, and Lee Lee are about to slave her until the real Ponypuff Pony appears.
Ponypuff Pony demands for them to let Dee Dee be free and threatens to eat them all if they disobey her, scaring the trio to give up and let Dee Dee be free.
The Ponypuff horse then tells Dee Dee to be free as the latter runs off in happiness.