Retro's Monster Island was mentioned as almost being finished. The Retro version of the area was originally imagined as being a giant monster itself, with eyes and other body parts being scattered across the environment, but this was scrapped later on. A new version of the area was later developed using the map data of the original version that was made by the Grigon developers.
Visit Monster Island on FF Legacies by talking to the Zombie Blowfish in Candy Cove!
In-Game screenshots of Monster Island
More "completed" version of Monster Island with cliff-side that never saw the light of day
Unity Screenshot from tail end of Monster Island development
Giant blowfish added by WifiSpy on the rock intended for the Mother Blowfish.
Legacies upsell for V.V. Argost missions about Monster Island
Legacies upsell for Drew Saturday mission on Monster Island
Hibagon in it's lair.
The Fusion Hominid monster fought in the TitanNano mission is internally known as Hibagon. The Hibagon was a cryptid from The Secret Saturdays. The monster bears a great resemblance to the cryptid. It would've appeared on Monster Island.
The Gatekeeper would have been the largest fusion monster to date. As it's name implies, it was meant to guard a gate on Monster Island. As a result, it had no walking or running animations. According to one of the developers for OpenFusion, the Gatekeeper is programed to take less damage when more players are near it. The monster has been hacked into the game multiple times.