Monsters are the primary antagonists of Among the Sleep and its prologue DLC. The original game features three monsters, while the DLC features two; these sets are exclusive.
The monsters' appearances are filtered through the imagination of The Toddler. As such, their appearances are relatively abstract and yet also quite simple.
Upon detecting a monster's presence, The Toddler will panic, manifesting as distorted vision and disturbing audio. If they make contact, The Toddler suffers a Game Over and must continue from an earlier portion of the stage. However, monsters are capable of sneaking up quietly behind the Toddler without triggering any of these cues, so he should always be aware in all directions.
Original game
The original game contains three monsters. All three of them represent The Mother, who is an alcoholic, at different stages of her drinking.
The Stranger (Harald)
The stranger resembles a silhouette tall figure wearing a coat & boots, it also holds a parasol. It represents The Mother donning a trench coat and leaving the house at night to purchase wine.
It shortly appeared as a shadow in the corner of the living room right before a lightning bolt struck. Because of her temporary absence, no monster attacks the boy early in the game. However, a jump scare of the Stranger occurs as a scripted event within the house and it might be able to catch it in The Playground.
The Stranger is the only monster in the DLC, but it has glowing eyes and has a black mass of smoke.
The Forest Lady (Hyda)
The Forest Lady appears in the second level, The Forest, it represents The Mother when she's drinking and in sorrow, it's the only monster that is able to walk on water and emerges from the library, you can hear it close by cricket like sounds.
It can shortly be seen in the fireplace room where you see it's head slowly going up and then down in the water.
The Child might have inspiration from this creature from the book, "Five Thirsty Animals", drilling for something to drink, Which also shows that wine is some kind of "bad water" thinking it will eat him.
The Coat Monster (Heap)
The Coat Monster appears in the third level, The Closet, and represents The Mother while in anger, as indicated by its hatred of loud sounds and the child making a mess.
It is a special among the monsters in that, along with capturing the Toddler and causing a Game Over, it will also throw things at him, slip into broken planks and will immediately teleport.
In the final game sequence, just as he is about to escape, it grabs him by the leg and seizes Teddy, causing his arm to be torn off. He later exits the closet at daytime and finds the Mother sobbing with an empty bottle of wine in one hand and Teddy in the other.
It is implied in the introductory sequence that he frequently hides in the closet from the Mother, and it is shown after completing the Closet level, that it indeed takes place hiding in the actual closet. This may cause her attempts to seize him to appear like the various coats in the closet are attacking him, explaining why this monster resembles a trench coat and why all such coats among the house appear so imposing.
Prologue DLC
The prologue DLC takes place in an unknown amount of time before the main game and features two monsters unique to itself.
The Shadow
While wandering the house, the Toddler must evade a shadowy silhouette vaguely similar to The Stranger. Its precise behavior and symbolism are unknown; however, it likely represents The Father in some way as he is the only one in the house and is known from the DLC to be aggressive.
The Furnace (Hons)
In the basement of the house is a large, retro-style furnace. Upon moving far enough into a dark spot in front of the Furnace, it will suddenly come alive, roaring and blazing, and chase after the boy with clumsy hops. If he then moves out of its activation zone and looks away, it will instantly return to its original space and wait to be awakened again.
In front of it are two light switches that can only be reached by climbing the nearby boxes. When he is within the light toggled by these switches, the Furnace returns to dormancy.
The Furnace can also be run straight past if he is quick enough. The reason for this is probably about how scary machines can be to other toddlers.