Monster Carnival

Monster Carnival Island was going to be an Island, but strangely disappeared from the map while it said "COMING SOON!". In it's place became 24 Carrot Island.

Disappearance
A few years ago, the Creators released a sneak peek of a concept art of what appeared to be a circus and stated that it was for an unfinished island which was presumably Monster Carnival Island.This shows that Monster Carnival Island was cancelled because it wasn't finished.

Beliefs of Return
Multiple times it has thought to be returning. Some people believed that Cryptids Island and Zomberry Island would actually be Monster Carnival. When a sneak peek of what seemed to be a carnival appeared on Daily Pop, some people assumed that it was for Monster Carnival Island. However, it was actually for the Carnival Costumes store on Night Watch Island. Whether it is returning is unknown.

There are also the "Fuzzy Monster" costumes in the store which might be hinting at something.

Found on the Creator's Blog
On the Creator's blog, there was a post by Captain Crawfish that showed the plans of what would have been on the island and would have had zombies. It was originally called Haunted Carnival Island. At the end of the blog post, Captain Crawfish quotes: "You never know, I guess.", hinting that it could be returning. There was also a small notepad for Monster Carnival Island.

Leaks
There were "leaks" of Monster Carnival island of clothes that weren't in the store or on an island, and in common rooms many people customized the clothes. Still, most of the leaks were patched.

Trivia

 * The username MonsterCarnival2012 is very briefly seen in the Poptropica Friends trailer. If you enter the username MonsterCarnival2012 on the Avatar Studio, a Poptropican called Monster Carnival will appear.