Duck Hunt

Duck Hunt is a duo made of the laughing hound and the duck from the Nintendo game named after the characters.

Beginnings

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.

Smash Bros.

The duo learned to cooperate and be a two character team when they showed up in Super Smash Bros. for 3DS / Wii U.

Trivia

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, tMoviehe other being R.O.B..