Big Yellow is the monster appearing in The Haunting Hour episode "Mascot".
For years, Big Yellow has been the Yellow Valley High mascot. He is shown to love being the school mascot and would do anything to keep his position. It's implied that he ate (or killed) the former team manager who got too close and told him he was no longer needed due to being a horrible dancer.
Big Yellow tried to defend his position once again when Willie and his friend named Drake were holding auditions for a new mascot. However, he was rejected by Willie (for the best), who wanted to make the new mascot named Wolfie. Big Yellow then ate Wolfie alive (off-screen) while he was walking home at night all by himself, somehow placed his severed head in Willie's bed, and sent Willie a message that said he wouldn't let anybody to replace him.
When Willie (who thought Big Yellow was just a student in a mascot costume) attempted to break into his dressing room/secret hideout in the school basement, Big Yellow tried to get rid of him by locking him in and unleashing a yellow-colored poisonous gas everywhere. However, Willie escaped and confronted Big Yellow in the gym all by himself. Big Yellow finally had enough and ate Willie alive (off-screen) when he had his back turned.
Drake found out what Big Yellow did (due to hearing Willie's phone inside his stomach) and couldn't do anything about it.