Monster-X is the mysterious Horror from HorrorLand, who has been secretly contacting Lizzy Morris and Luke Morris. Monster-X is not Byron, but actually Ned, who is also a horror in HorrorLand. This is made clear in The Streets of Panic Park. He is mentioned on the blog on EscapeHorrorLand.com and in the Horrorland Survival Guide. He is also the Horror who gave them the blank card, which was originally Britney Crosby's.
Ned also appears 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.