Yenndo would usually appear in Funtime Auditorium on Night 3, the player would rarely encounter Yenndo instead of Funtime Foxy.
In Custom Night
In Custom Night, Yenndo acts very similar to Golden Freddy from the first and second game. At random times, Yenndo will teleport on the right side of the office after closing the monitor even if both doors were closed. The moment the player sees him, they must raise the monitor in order to stop him from taking away the oxygen which allows Yenndo to kill him.
Yenndo appears as a endoskeleton of Funtime Freddy with the exception of his microphone and Bon-Bon on his hand.
Yenndo's existence remains a mystery, it maybe assumed that Yenndo might be the endoskeleton of a animatronic, possibly Funtime Golden Freddy/Funtime Fredbear and resides in the building
Five Nights at Freddy's 4
Ultimate Custom Night
Five Nights at Freddy's Novel Series