Denizen

The Denizens are immensely 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 hoard, to be released upon their defeat and used for the Ultimate Alchemy. It is quite possible that, like the imps, the Denizens will reflect the children's Associated Items in some way. Each of the kids' Denizens also reflects the web browser they use. It is interesting to note that each player's denizen corresponds to the Web Browser the player uses.

Typheus
Also referred to as the "The Slumbering One", Typheus is the Denizen of the Land of Wind and Shade, who rests in a castle. He is responsible for flooding the Land with Underlings and Oil, blocking off some Parcel Pyxides. Terezi was able to create an alternate timeline by directing John to bypass the Seven Gates and attempt to kill the Slumbering One in its sleep, a plan which was intended to fail catastrophically, resulting in John's demise.

Somehow, a minion of Typheus seems to have HS 2757 (ended up) in Jade's House.

In Greek myth, Typheus was a terrible monster that even the Gods feared, and after he was defeated, was trapped underground. (Specifically, under Mount Etna, the volcano Hephaestus was said to reside in.)

Cetus
The Denizen of the Land of Light and Rain. Not much is known about her except that it has eaten every living thing in the ocean and subsequently taken a nap. It is likely aquatic. Curiously, the meteor carrying baby Rose also killed all aquatic life in a nearby lake.

In Greek myth, a Cetus was a large fish, serpent, or sea monster, which would explain all of the fish being eaten.

Hephaestus
The Denizen of the Land of Heat and Clockwork is Hephaestus, lord of the forge.

In Greek myth, Hephaestus lived in a volcano, and was the God of blacksmithing. In the timeline of both Alpha Dave and Alternate Future Dave, Hephaestus became enraged when Dave broke Caledfwlch, and even more enraged when it became apparent that Jade wouldn't be entering the Medium, as he desired access to her Volcano, the Kids' session's Forge, to repair Caledfwlch.

Echidna
Jade's Denizen is Echidna, referred to as the "Mother of all monsters", and mate to Typheus. This may be related to the fact that Jade lives next to the kids' session's Forge. Other than that, nothing is known about her.