They started out as enemies. The dog will do his best to catch the shot ducks for the hunter, but if it doesn't happen the dog will laugh at the hunter.
Super Smash Bros. series
Super Smash Bros. for 3DS/Super Smash Bros. for Wii U
The duo learned to cooperate and be a two character team when they showed up in Super Smash Bros. for 3DS / Wii U.
Super Smash Bros. Ultimate
They are unlockable characters in Super Smash Bros. Ultimate also classified as fighter #59.
Duck Season Form
Duck Hunt in Super Smash Bros. Wii U/3DS
Dog's infamously laughing at the player for missing the ducks
Duck Hunt Dog's Taunt
Despite being video game characters, their game started out as a toy, back when Nintendo used to make toys before they moved to video games.
In Europe, the character(s) are referred to as "Duck Hunt Duo".
Masahiro Sakurai, the creator of Super Smash Bros., has said there is actually a third character in the duo. The third character is the hunter that stands behind the screen. This is also revealed if Palutena's Guidance discusses the respective fighters.
The Dog is like the Nintendo version of Muttley from the Hanna Barbera universe.
The Dog is the only character in the Super Smash Bros. series where all of his alternate costumes are based on real-life dog breeds.
The Dog is one of the two characters in the Super Smash Bros. series where their alternate costumes are based on non-gaming characters, the other being R.O.B..