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. While Valve had anticipated Hoopy the Hoop would become an internet meme, 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 first appears in Portal 2 underneath a floor panel in Test Chamber 2. He makes his second appearence behind a desk in Test Chamber 6; this is the same den as referred to by the Final Transmission achievement.