Hoopy the Hoop/ko

In Portal, Hoopy the Hoop was a hoop fallen from the sky

As stated in an interview with Valve on "Opening The Portal: Exploring The Game's Development", they never expected "The Cake is a Lie" to become such a huge hit. The developers expected, if anything, that Hoopy the Hoop would become the meme derived from Portal.

Erik Wolpaw explains, "We thought we should have a warehouse full of Hoopy t-shirts and mugs and posters…we would watch that hoop roll by over and over again. That was the part of the game we were most proud of, and nobody cared."

Portal
Hoopy the Hoop rolls across the screen

Portal 2
Hoopy is known to appear in a few places within Portal 2. Its earliest known appearance is in GLaDOS' ruined chamber, where it lays slightly to the left of what could be described as GLaDOS' head; its second known appearance is underneath a floor panel in Test Chamber 02. Hoopy's fourth appearance is in multiple custom test chambers (Created and published with the Perpetual Testing Initiative.) It is seen by noclipping into the ceilings of the exit rooms, although it doesn't appear in each one.