Monster-X is the mysterious Horror from HorrorLand, who has been contacting Lizzy Morris and Luke Morris very secretly. He is mentioned on the blog on EscapeHorrorLand.com. He is also the Horror who gave them the blank card which was originally for Britney Crosby, but it ended up to the siblings. Monster-X is not Byron, but actually Ned who is also a horror in Horrorland. This is told at the end of The Streets Of Panic Park.
He also appeared in Say Cheese - And Die Screaming! as the Horror guarding the front gate and when the Very Special Guests tried sneaking out of the front gate, they ended up nearly dying from being strangled by purple vines. Ned was secretly trying to protect the Very Special Guests from going to Panic Park. He also did the same thing in Welcome to Camp Slither.