Undodog(also formatted as Undo Dog) is a helpful character first appearing in Mario Paint, selectable as an icon in the bottom toolbox in the Art and Music modes. He is a tan dog that has a rounded white box around his neck like a collar, most likely representing his icon. His name is a pun on the word "underdog", and a portmanteau of the words "undo" and "dog", and, as his name states, he helps the player undo the last action.
Undodog debuts in Mario Paint. His appearance in this game is that of a tan dog with a thin black box around his neck, like a collar, probably representing the outline of his icon. He will also dance and jump around at the bottom of the screen when the player is idle in the stamp editor or drawing board color selection. He will quickly run back to his original location if the player moves the mouse. If the player turns the game's music off and leaves it silent, Undodog will sometimes sneeze loudly. According to the instruction manual for the game, this is because of his allergies. 
Super Mario series
Super Mario Maker/Super Mario Maker for Nintendo 3DS
He reappears in Super Mario Maker and Super Mario Maker for Nintendo 3DS with the same function as his original appearance: reverting the player's last edit whenever his icon is tapped. In this appearance, he wears a white box resembling an Elizabethan collar instead of a black box. He also appears throughout the game outside of the course editor such as in the game's manual with Mary O. and Yamamura, riding a rocket in the background in Course World and when a player completes the 10-Mario or 100-Mario Challenges, and in the corner during Gnat Attack. When 10 seconds remain on the timer in Gnat Attack, he'll start barking and jumping excitedly. If the player shakes the heart SFX, its icon will turn into Undodog wearing a surgical face mask. This SFX will cause static to appear briefly with a buzzing sound followed by the course's music turned off temporarily (or permanently, if it is equipped to Mario). The icon and the music being turned off is a throwback to Mario Paint where the bottom lever in the background music option has a similar appearance and effect when selected. A special Event Course titled "Walkin' with Undodog" also exists.
Super Mario Maker 2
Undodog reappears in Super Mario Maker 2 filling similar roles and having the same appearance as in Super Mario Maker. However, he also has a larger role in the game's story mode (appearing for the first time as a 3D model), providing Mario access to various jobs to help rebuild Peach's Castle. At the beginning of the story mode, Undodog excitedly jumps around and accidentally presses the Reset Rocket button on the ground, completely erasing Peach's Castle. At the end, after the castle is entirely rebuilt, Undodog goes over to the Reset Rocket again and looks at it happily while everyone gasps off-screen. It is unknown if Undodog erased the castle again or left the Reset Rocket alone. Undodog can also be seen in the background of nighttime levels in most themes riding the Reset Rocket.
Undodog talks to Mario as if he is a pet and even refers to Mario as a "good boy", while on the other hand Mario talks to Undodog by saying phrases such as "woof" and "bark". Additionally, after completing Story Mode, Undodog will tell the player jokes for 100 coins each. Ten jokes in total are told, with most referencing several in-game Course Maker elements, highlighted in blue in the jokes. Once Undodog runs out of jokes, the player can hear a previous one again for free, chosen at random.
- "Why are ? Blocks so good at holding power-ups? Because they have so mushroom inside!"
- "Where do Bowser's minions keep their chickens? In a chicken koopa!"
- "What do you call a sad Dry Bones? A Cry Bones!"
- "Whaddaya call a four-legged ghost with antlers? A Scare-A-Boo!"
- "Why didn't Mario enter the Pipe in the desert? Because it was PIPING HOT!"
- "Why is the Angry Sun so angry? Somebody ate all of its desert!"
- "What do you call an aspiring Mario Maker? An up-and-plumber!"
- "I just told some of my jokes at a Ghost House. They kept shouting "BOO! BOOOOOOO!"
- "Why didn't Bowser replace his cracked fishbowl? He's too Cheep Cheep!"
- "Why did the Lakitu record itself catching a squid? For the Blooper reel!"
Super Mario Maker for Nintendo 3DS
- Digital Manual Description: "Each time you tap this icon, your last action will be undone through the power of Undodog."
Super Mario Maker 2
- "Woof... (It was an accident...)"
- "Woof? Woof... (Seen that button around? I wanna jump on it again...)"
- "Bark bark!"
- "Bark! Bark bark! (Oh, you're rebuilding it! Good for you!)"
- "Bark bark! (I have jobs!)"
- "Bark! (Why thank you!)"
- "Woof woooof?! (Who's a good boy?!)"
- "Bark bark! (Hey, I have another job for you!)"
- "Bark bark! (Hey! I've got another job for you.)"
- "Bark bark! (Thanks for helping me out with those jobs!)"
- The Mario Artist series also uses a dog's head as the "undo" icon; however, the dog bears more resemblance to a beagle than it does to Undodog and may not be the same character.
- While Undodog himself does not appear in Super Smash Bros. for Nintendo 3DS/Wii U, his sneezing sound effect is incorporated into the "Mario Paint Medley" music track in the Wii U version.
- When Undodog is telling a joke in Super Mario Maker 2, the music fades out. Afterward, audience laughter can be heard, which is the same audio as the "Guffaw" sound effect.
- ↑ "When you click on the icon shaped like a treble clef, you can access a control panel that will let you change the background music. In this section, you can choose one of three different tunes. The fourth lever lets you work inside a quiet working environment. Don't be surprised, however, if Undodog's allergies cause him to sneeze occasionally." - Mario Paint US instruction manual, page 19
- ↑ Nintendo (June 12, 2019). Super Mario Maker 2 Gameplay Pt. 3 - Nintendo Treehouse: Live | E3 2019. YouTube. Retrieved June 13, 2019.