Yellow mutt is one of Easter eggs from playtime with percy created by FazieFunbear/fazie funbear he is fangame based on Golden freddy (from five nights at freddy's 1) for a new character for this game

Yellow Mutt is an older model Percy Poodle from a now-unnamed playhouse similar to Percy's Playhouse. He is only seen in-game very rarely. Consider yourself lucky if you ever see him! You won't live to see it again

Yellow Mutt's Bonuses description

Yellow Mutt is a character created by DaNoodle, only made as a very hard-to-come-by easter egg. Did you know Yellow Mutt is actually the first ever built animatronic by Paltronics? Does it have any relevance to the plot? Probably not! But then again, things could always change...

Yellow Mutt is an uncommon animatronic that has the potential to emerge at any given moment on any night. According to lead developer of Playtime with Percy, he has a 1/10,000 chance to spawn whenever pulling up your Pal-Boy or Pal Percy.

Behavior

Similarly to Sunny Sheep, Yellow Mutt might occasionally be discovered slumped over in your office after pulling down your Pal Percy or Pal-Boy. In the event that Yellow Mutt manifests, all efforts to counter its presence are rendered futile, as a depiction of its face will superimpose over Nick's point of view, showcasing its white eyes and teeth in the final frame

However Y. Mutt is appear on a image of all animatronics "thank you" seen he's behind the animatronic his head was here/there