Endogeny (known in battle simply as "Amalgamate" before its spare conditions are met) is an Amalgamate found in the True Lab, where it is first disguised as floating debris in the fan room. It is the first Amalgamate to be introduced to the protagonist as one.
Endogeny is composed of the Determination-filled souls of several dog monsters, including the parents of Dogamy and Dogaressa and other miscellaneous family members of Doggo, Greater Dog, and Lesser Dog.
Endogeny takes the vague shape of a large canine, with two pointed ears atop its "head" where instead of a face it only has a single orifice. Its wide body has six digitigrade legs, which form the silhouettes of five smaller canines between them. The silhouettes between its legs gain satisfied faces when it can be spared.
Endogeny displays the same excited behavior many of the game's dogs do, but seems to be unaware of the damaging nature of its enormous weight and powerful body, much like the Greater Dog. It exudes a fluid from its single orifice, which becomes a "happiness froth" when the Endogeny is excited.
- A sprite resembling the Annoying Dog (but with the Endogeny's orifice for a face) retracts its legs and reveals a rocket underneath its tail, using it to fly towards the SOUL in a straight line several times before the attack ends. This attack is capable of exiting the boundaries of the Bullet Board.
- The dog appears to the right of the screen, first faceless but then developing the orifice. Then it begins to move up and down while firing backward-facing arrows towards the SOUL in a random pattern.
- Endogeny can be spared by beckoning, petting, playing and then petting again twice (identically to how Greater Dog is spared).
- Unlike the Greater Dog, ignoring Endogeny will not have any effect.
- The Hush Puppy can be used to pacify and spare the Endogeny immediately, as can the Stick (in a similar fashion to other dog monsters).
- After using the Stick or Hush Puppy, however, the regular set of ACTs will have to be carried out to achieve the yellow text in the credits for the Amalgamates (though using one of these items first makes this task easier, as it prevents Endogeny from attacking.)
At the end of the True Pacifist Route, it can be seen with most of the other dog characters, revealed to be its family, outside the Snowdin Library. When interacted with it imitates the signature sound and (perhaps weakly) facial expression of Greater Dog, who probably inherited these traits from one of the members of Endogeny. Its yellow text in the credits lists it, along with the other Amalgamates, as living with its family, implying it continued to spend its time with the other dogs.
- It's unclear how many dogs this counts as. [Check]
- It's the Amalgamate. [Encounter]
- Amalgamate is watching you intently. [Neutral]
- It smells like a bunch of dogs. [Neutral]
- You threw the stick and the dog ran to get it. You played fetch for a while. [Use Stick]
- You try looking away, but it seems to appear everywhere you look. [Ignore]
- You call the Amalgamate. | It bounds towards you, flecking a strange liquid from an orifice. [Beckon]
- Amalgamate is hovering close, looking for affection. [After Beckon]
- You pet the Amalgamate. | It convulses rapidly... then calms down. | It rests quietly on your lap for a moment... | Zzzzz... | Suddenly, it shoots away and crawls wildly on the walls! [Pet]
- Amalgamate is striking the wall with its claws. [after Pet]
- You throw your weapon into the corner of the room. | The Amalgamate brings it back to you... Proudly? | You repeat this process a few times. | Now Amalgamate is very tired... It leans its dripping, amorphous body on you... [Play]
- Amalgamate is twitching affectionately. [after Play]
- You pet the Amalgamate. It starts to generate a Stage I Happiness Froth. [Pet]
- Amalgamate's convulsions intensify. [after Pet]
- You pet decisively. The Amalgamate seems to be satisfied by all this. [Pet]
- Endogeny is contented. [Meeting Spare conditions]
- You'd give it a tummy rub, but it's not clear where its tummy begins or ends. [Petting after meeting Spare conditions]
- Amalgamate seems too tired to play. [Playing after meeting Spare conditions]
- The Amalgamate's feelers rotate quickly. Nothing else happens. [Beckon after meeting Spare conditions]
- The name Endogeny may come from the word endogenous, which means "having an internal cause or origin" and refers to the Amalgamate containing multiple members of a single family (and possibly hinting at it containing the parents of some of its own components). Like all other dog characters in its family, Endogeny contains the word dog within its name.
- Endogeny's theme before meeting spare conditions is titled "
mus_woofenstein" in the game's files, this name is presumed to be a dog-related pun on Frankenstein and/or the game series Wolfenstein.
- The nature of one of its attacks combined with some of its flavor texts might suggest it possesses some form of propulsion on its body, such as a rocket.
- A comment by Toby Fox on the Steam page says, "If you play this game, can you count the dogs for me? I'm not very good at it." This is likely a reference (possibly partial, given the number of other dogs featured) to Endogeny's Check text.
- The closer you get to sparing Endogeny, the faster the battle music plays. It stops increasing in speed once Endogeny can be spared and is overlapped by a slowed-down version of Dogsong.
- Endogeny is the only amalgamate that looks similar to its overworld sprite.
- If the protagonist uses the Hush Puppy during battle, it will cause Endogeny to fade out prematurely before its contented version is shown.
- During the battle, Endogeny will vibrate, gradually growing more intense as spare conditions come closer. Their orifice on their face will also open and close faster.
- The dog that Endogeny summons as an attack is called "nofacedog" in the game's files.
- The sound that plays in Endogeny's battle is entitled "mus_disturbing.ogg". It also plays in the room with the Amalgamate in the shower.