The Annoying Dog is a small, white, smiling dog representing the game's creator, Toby Fox.
The Annoying Dog shows up periodically throughout the game, most often around Papyrus. Whenever it shows up, it often somehow comedically ruins the characters' plans.
Altered or Corrupted SAVE Files
Altering "file0" and "undertale.ini" variables to an illogical combination result in the Annoying Dog appearing upon game launch. There are two versions of this screen:
- Dance of Dog plays as the Annoying Dog runs in place mid-screen. The song may change pitch upon closing and reopening the game. 
- Sigh of Dog plays, and the Annoying Dog is sleeping in the center of the screen.
The "Dogcheck" value checks for a valid SAVE and room combination and, if the value returns out of range, the Annoying Dog appears, and the player cannot progress. The player can enter some inaccessible rooms after using a hexadecimal editor to change the "Dogcheck" value.
The Gauntlet of Deadly Terror
Papyrus Boss Fight
The Annoying Dog appears during the "special attack" sequence, munching on Papyrus's special "Bone." After Papyrus gets angry at the Annoying Dog, it exits the bullet board, dragging the bone with it. It appears again when Papyrus does a "cool regular attack."
Papyrus and Sans's House
If the protagonist interacts with the sink in the kitchen, Papyrus comments on how he raised the sink so he could fit more bones beneath it. He then invites the protagonist to check inside. The cabinet door then opens to reveal the Annoying Dog chewing on one of the bones, before it runs out of the house as Papyrus shouts to get it. Interacting with the sink again will lead to the Dog Shrine if playing the PS4 or Nintendo Switch versions of Undertale.
Mettaton's Third Encounter
During the bomb defusal sequence, the Annoying Dog is a bomb that the protagonist can defuse. Its check text reads "blissfully unaware of its circumstance." When defused, the narration reads "Dog defused!" instead of "Bomb defused!", which is unique to the Annoying Dog.
The Annoying Dog appears after completing the Piano Puzzle. Playing the right notes opens a secret door to the Artifact Room. Once the protagonist enters the room, the Annoying Dog appears in their inventory. While trying to grab the artifact (with the Annoying Dog in the inventory), a message appears saying "you're carrying too many dogs." Using/Dropping the Dog summons the Annoying Dog beside the protagonist. The Annoying Dog then runs to the Legendary Artifact and absorbs it, while Dogsong plays once. Shortly after, the Annoying Dog runs out through the walls out of the frame. Dog Residue is then left in the inventory.
THERE'S SOMETHING ANNOYING ABOUT THIS ROOM.Papyrus when called from the Piano Puzzle room.
BUT I CAN'T QUITE PLACE MY FINGER ON IT...
Calling Toriel's phone when the Annoying Dog is in the inventory results in Toriel's phone ringing in the protagonist's inventory.
It is impossible to put the Annoying Dog into a box, as trying to use a Dimensional Box through the cell phone displays the text "The box is aclog with the hair of a dog." Leaving the room with the Annoying Dog still in the inventory makes it disappear until reentering. Though normally the dog can only be in the inventory in the Artifact Room and Piano Room, editing "file0" allows the protagonist to have it in their inventory. If removed with file manipulation, attempting to take the artifact will display the message “This will never happen”, after which the artifact stays on the stand, but says that it’s gone.
If added to the inventory through file manipulation, the protagonist can sell the Annoying Dog at the Tem Shop for 999G and, if Temmie denies the sale, she negotiates the prices of 1251G. After negotiation, the Annoying Dog is the most valuable item in the game.
Nothing happens if the protagonist uses or drops the Annoying Dog outside of the Artifact Room.
After Toriel's boss battle, the Annoying Dog arrives and announces the end of hard mode, much to Toriel's displeasure. When it speaks, its text moves in place with a wave effect. After its dialogue, the game restarts.
True Pacifist Route
The Annoying Dog appears on the ending screen, stays below the "THE END" text, and eventually falls asleep. It does not appear if the protagonist completed a Genocide Route before.
In fights with most of the dog members of the Royal Guard in Snowdin, the Annoying Dog may appear during the enemy's turn to represent the monsters themselves. Doggo however, does not use the Annoying Dog in battle.
- The Annoying Dog appears in the Joystick Config with an object atop its head related to the season (except Summer), along with the Uwa!! tracks and while accompanying snow, leaves, and the sun.
- When the protagonist slides past a thick row of trees after the last puzzle before Snowdin, a snow sculpture appears on their head. This sculpture sometimes resembles the Annoying Dog.
- Gyftrot is sometimes decorated with an Annoying Dog.
- The protagonist sees The Annoying Dog's silhouette on the radar that shows up during the date with Papyrus.
- A framed picture in the Tem Shop portrays the Annoying Dog.
- The Annoying Dog hides in the bone drawer in Undyne's House.
- The protagonist can occasionally see the Dog's face on the front of the River Person's boat.
- Calling Papyrus near the L2 Elevator in Hotland causes him to mention a certain dog stealing his lasagna.
- During the MTT News Report, a building has the Annoying Dog's silhouette on top of it.
- When the protagonist is at Mettaton's colored-tile puzzle, repeatedly triggering the piranhas makes the dog appear in the water for a short moment.
- The protagonist can interact with the Annoying Dog through the right-most door in the east branch of MTT Resort. When using a Hot Dog...?, Dog Residue, Dog Salad, or certain other dog items, unique dialogue appears and the protagonist can gain a Hush Puppy.
- Putting a Hot Cat in front of the door causes the dog to growl at it and does not accept it.
- Calling Papyrus in this area also causes the Annoying Dog to bark behind the door, provoking a surprised reaction from Papyrus upon calling him again.
- If the protagonist kills all the major bosses (Toriel, Papyrus, Undyne, and Mettaton) during a Neutral Route and has killed nobody else, during his phone call, Sans tells them that a small white dog became the president and that this is best for everyone.
- The Annoying Dog can be found in the room behind the Mysterious Door in Snowdin Forest.
- The protagonist can see the Dog in the trash tornado in Sans's room on rare occasions.
- The protagonist can see the Annoying Dog on the left side of the garbage dump during Alphys's date.
- In the True Pacifist Ending Credits, the Annoying Dog is usually seen sticking out of Gyftrot's right antler tree.
- The Annoying Dog represents Toby Fox himself; the Steam Greenlight trailer and the original Kickstarter video show the Annoying Dog stating this.
- In the game files, some sprites of the Annoying Dog contain the word "tobydog."
- The Annoying Dog steals Toriel's phone if the protagonist waits for her in the hallway of the Ruins. This is likely why Toriel does not pick up the phone until the epilogue.
- However, after the protagonist advances to the very next room, Toriel talks normally on the phone. It is possible that the Annoying Dog stole the phone again after Toriel's battle, or it simply ran out of batteries.
- If Papyrus is called in the Artifact Room, he notes there is something annoying about the room but cannot identify what, referring to the Annoying Dog.
- The sound effect used for The Annoying Dog first originated as a sound effect for Mario Paint.
- Through save file manipulation, it is possible to deploy multiple Annoying Dogs in the Artifact Room. Even if the artifact is not present, they approach the podium and the message saying that they absorbed the artifact displays.
- When talking to Bratty and Catty at the end of the True Pacifist Route, Catty mentions a "cat" she saw, and describes it as little and white. Bratty then corrects Catty, saying it was a dog. It is most likely this dog was actually the Annoying Dog.
- While his breed hasn't been confirmed yet, the Annoying Dog strongly resembles a Samoyed, with pointed ears, white fur, and a perpetual 'smile.' Others say that the dog resembles a white Pomeranian. However, the latter is more likely, as it's referenced as a "tinypom" in the game files for the Demo, its walking sprite is named "spr_tinypomwalk", and its bark "snd_pombark", for example.
- The Annoying Dog is similar to Ruffini the dog from EarthBound, as both Annoying Dog and Ruffini are based on the game designers.
- Originally, if the player beat the game fast enough, the Annoying Dog wearing a bikini would appear at the ending screen. However, it didn't make it into the final game. This references the Metroid series, where the main character, Samus Aran, wears a bikini if the game is completed fast enough.
- The Annoying Dog flying through the wall by ignoring the collision may be a refrence to the Steam Greenlight trailer where the message telling you to not go into the Artifact Room because there's no collision yet.
- ↑ Undertale corrupted/broken save jingle 1. October 21, 2015. YouTube
- ↑ Undertale corrupted/broken save jingle 2. October 21, 2015. YouTube.
- ↑ Undertale - Disabling Dogcheck (The Annoying Dog Error Room) YouTube
- ↑ "BUT WHAT DOES THE L STAND FOR? LASAGNA? I HOPE NOT. I DON'T MAKE LASAGNA ANYMORE. THAT GLUTTONOUS DOG ALWAYS EATS IT FIRST!" - Papyrus
- ↑ "a small, white dog became president of the underground. it sleeps on the throne and does absolutely nothing. strangely, it seems this is the best life for everyone. thanks." - Sans
- ↑ UNDERTALE ORIGINAL KICKSTARTER TRAILER
- ↑ "Bark! Bark! Bark... Bark! (You hear a distant voice.) Stop, please! Come back here with my CELLPHONE!" - Flavor Text if the protagonist waits in the initial room that Toriel tells them to.
- ↑ HMMM... THERE'S SOMETHING ANNOYING ABOUT THIS ROOM.
BUT I CAN'T QUITE PLACE MY FINGER ON IT... THEN MY PLACE MY WHOLE HAND ON IT... THEN PET IT AFFECTIONATELY... - Papyrus
- ↑ Originally I was going to make it if you beat the game fast enough, you’d be able to see the annoying dog in a bikini. - Undertale Tumblr
- ↑ Completion screen (bikini dog)