Danny Phantom - Episode #15 "Public Enemies", first aired 4 February 2005, Nickelodeon.
This morning my sons were watching "Danny Phantom" on television. The youngest, who has been learning a bit of Esperanto, yelled "Daddy, Daddy, they're talking Esperanto!" and sure enough, they were! Wulf, also known as the "Esperanto beast", is a large ghost wolf who wears green and speaks Esperanto. The only downside I noticed is that Esperanto is introduced as a "secret language for geeks" or some such thing. I can't vouch for all the Esperanto because I couldn't pay close enough attention, too much other stuff going on then, but what I heard sounded good for the most part— actual Esperanto, except maybe for one bit that I couldn't quite make out.