Hoopy the Hoop

In the first Portal, Hoopy the Hoop was a lone hoop that dropped and rolled over in-front of the player at the end of the game. Valve had anticipated Hoopy the Hoop would become an internet meme, instead Cake took that place and Hoopy the Hoop was forgotten.

Portal
Hoopy the Hoop rolls across the screen at the end of the game

Portal 2
Hoopy appears in Portal 2 behind a desk in Rat Man's den in Test Chamber 06. It is the same den as referred to by the Final Transmission achievement.