Hoopy the Hoop

In the first Portal, Hoopy the Hoop was a lone hoop that rolled past the screen at the end of the game. Valve had anticipated Hoopy the Hoop would become an 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 one of Rat Man's dens. It is the same den as the one referred to by the Final Transmission achievement.