Denizen

The Denizens are powerful enemies, one of which is found in each of the four lands. They are, at first, asleep, but as the player progresses through the Seven Gates, they awaken. Each is said to guard an immense Grist horde, to be released upon their defeat. It is quite possible that, like the imps, the Denizens will reflect the children's Associated Items in some way.

John's Denizen
Referred to as the Slumbering One, the Denizen of the Land of Wind and Shade rests in a castle. It is responsible for flooding the Land with Imps and Oil, blocking off the Parcel Pyxis. In an alternate timeline, GallowsCalibrator directed John to bypass the Seven Gates and attempt to kill the Slumbering One in its sleep. This plan failed catastrophically, however, resulting in John's demise. Rose and Dave suspect that John's death was intentionally orchestrated by the Trolls. Some theorize that the beast Dad is seen fighting in the one-year anniversary Flash update is John's Denizen, but given that the fight apparently takes place before John descended to the planet's surface, this is unlikely-- if the Denizen were already dead, then it would not have been able to kill John in the alternate timeline.

Rose's Denizen
Not much is known about the Denizen of the Land of Light and Rain, except that it has eaten every living thing in the ocean, and subsequently taken a nap. Mom is seen engaging a creature that may be this Denizen, and she presumably won.

Dave's Denizen
If the monster Bro has begun fighting is Dave's denizen than it appears to be a large yellow lava octopus of some kind. It is similar in appearance to the Kraken of Norse mythology.

Jade's Denizen
We have not encountered Jade's Denizen yet since she has not yet entered the Medium.

The Seventh Gate
It is speculated, given the round robin nature of the gate hopping, that John will battle Jade's Denizen. His first gate lead further into his Land, where he found the second gate, taking him to Rose's Land. If he builds up to her third gate further into her Land, he'll find the fourth gate, leading to Dave's, and so on. It is unknown if the other three kids will take similiar journeys, but if they do, then Rose would fight John's Denizen after ascending to his seventh gate, Dave would fight Rose's Denizen, and Jade would battle Dave's. Each child would need to be constantly building to prevent another falling to their deaths, of course, assuming these journeys are simultaneous.