Secret Bosses
In the case that the player lacks space for the Dealmaker or Puppet Scarf, Spamton will do this after reacting angrily
Summon a chest
This Undertale fan character originated from someone's dream
Prunsel
This is Sans' skin color
None
Cat Petterz, the game FRIEND is theorized to originate from, was clearly based off this real game
Catz / Dogz
This joke theory suggested that Chapter 3 would go much different than we were lead to believe
Despite seeming like nonsense at first, PLUEY actually means this
Rain in French
According to an alleged story, an fan was once given this altered food item at a convention and was harmed by it
Razor Cookie
After completing the Genocide Route, instead of the regular ending photo, this is shown instead
A version of it with everyone's faces crossed out
Despite being called something different in Japan, this game was marketed as Super Mario Bros. 2 in the west
Doki Doki Panic
This take on Woody Theory combined both it and the idea that Flowey would return as the secret boss of Chapter 3
Petaly
Even before becoming a secret boss, Gerson in Undertale seemingly had this piece of fourth wall-breaking knowledge
He knew the player couldn't kill him
This old AU followed Sans after Frisk repeated hundreds of Genocide runs
Dusttale
This backer character was the only one of them to not get into any controversy
Glyde
Despite not seeming to have a connection, this character's laugh is an altered version of FRIEND's
The Knight
Despite being called something different in Japan, this game was marketed as Super Mario Bros. 2 in the west
Doki Doki Panic
This part of the Knight's design seemingly confirms a fan theory regarding Gaster
This old AU character was a variant of Frisk who fell to the same fate as Gaster
Core Frisk
Despite DELTARUNE lacking the character of Clam Girl, this other similar clam appears
People speculate that it was through this that we first saw the Forgotten Man's dialogue
Valentine's Day Card
This multi-part theory is arguably the longest one ever, at multiple hours to watch the whole thing on YouTube
Device Theory
In order, this is the solution for one of the puzzles that gives you one of Jevil's broken keys
Diamond, Heart, Club, Spade
This fan route that originated from someone's dream followed Spamton as the player manipulated him into killing people
Salt Route
Snowdinn Innkeeper
These three songs are also present in the sound test along with Gaster's theme. All of them.
This theory, while being proven mostly false, correctly predicted that a certain Undertale character would reappear as a Darkner
Oberon Smog