Billy is a main character in The Grim Adventures of Billy & Mandy.
Cartoon Network Racing series[]
Cartoon Network Racing: Adult Swim Edition[]
Billy makes a cameo on a poster, along with Mandy and Grim, on the "Dunder Mifflin Parking Lot" track. It appears to be advertising their show. It is hard to see normally, as it is right behind the fourth corner of the Dunder Mifflin building and usually the player is going too fast to notice.
Future[]
Billy is going to appear in future installments of the Cartoon Network Racing series.