A moveset swap is when hacking causes a character to take on the animations and moveset of another character. This is doable in both Super Smash Bros. Melee and Super Smash Bros. Brawl. Results vary; some characters can impersonate others with no trouble (most commonly clones or semi-clones, such as Fox and Falco), while some combinations are unstable, freezing the game when some moves are used (or even as soon as the match is begun).
In Melee, moveswapped characters retain their body proportions and simply rotate their bones to match the new animations; for example, Donkey Kong's legs would pass through platforms if he was given the moveset of someone like Jigglypuff. However, in Brawl, characters change body shape to match their new animations, leading to unique distortions; for example, a character with short legs such as Mario would have his limbs stretched if given the moveset of a tall character such as Snake.
If the character used has loose hair, a dress, or the like, its will remain stiff if the animations do not include it. This extends to all parts of a character; for example Mario's ring fingers will remain "closed" when using Yoshi's three-fingered animations.
When a moveswap is used, most standard moves retain their characteristics from their original character. For example, Yoshi can use Luigi's forward smash without difficulty. However, more complicated moves may result in a "hybrid" move, such as using a Stall-Then-Fall when the original character does not have one. Special moves in particular have unpredictable effects, ranging from a surprisingly glitchless combination to a character-freezing situation (such as Mario, using Skull Bash, flying sideways at a constant speed until he dies).
In Melee, hitboxes would shift to accommodate the new moveset. In Brawl, hitboxes remain on the same body part. For example, Yoshi using Luigi's forward smash will still have the hitbox attached to his head (where it would be normally for Yoshi's forward smash).
These moveset swaps have gained a sort of popularity or recognition. When the code for swapping movesets first made its debut, several examples were compiled together in a few YouTube videos, sparking the phenomenon. The moveset swaps featured in the videos have since become the most famous.
Manchu (Pikachu as Ganondorf)
Manchu, also known as "Pikadorf," "Ganonchu," "Longchu," "Freakachu," or "Pikaman" or "Devil-chu" (due to the humanlike proportions), is a moveset swap in which Pikachu is the character model and Ganondorf's moveset is used. This is perhaps the most famous moveset swap, with Moveset hacking being used to make a moveset for the model, seen here.
Manly-Man (Snake as Captain Falcon)
Manly-Man's moves cannot be used as it crashes the game. The name is derived from the popular meme that both Captain Falcon and Snake are incredibly manly, so a hybrid would result in a manlier man.
Mariolimar (Mario as Olimar)
Mariolimar, also known as "Mini-Mario" and "Olimario", is a moveset swap in which Mario is the character model and the moveset is Olimar's. He looks squashed, and has a humorous miniature appearance.
Roadkill (Sonic as Fox)
Roadkill is the name of the moveset swap in which Sonic is the character model and Fox is the moveset. The name is derived from the fact that Sonic has his eyes closed the entire time and his arm in a fixed position, making it similar to "roadkill." For unknown reasons when Roadkill uses his Up Special Move it creates a golden Arwing in place of Sonic's spring. Note that other move swaps with Sonic can give him the appearance of having his eyes closed.
Wolvereach (Peach as Sonic)
Wolvereach, also known as "Screach/Screech" or "Nightmare Peach" is the moveset swap in which Peach is the character model and Sonic is the moveset. Wolvereach is the combination of the words "Wolverine" and "Peach" and is pronounced wolver-reach. The reason why this name was given to this hybrid is because of the long "claws".
Wolfshi (Wolf as Yoshi)
Other Famous Moveset Swaps
As well as the given examples, several other moveset swaps have been seen. For example, one is of Toon Link given Meta Knight's moveset, in which he loses most of his lower waist and down and walks around on two feet loosely connected to his upper waist. In another swap, Mario is given Luigi's moveset, making his nose smaller and face shriveled up. Mr. Game and Watch was given R.O.B.'s, causing his arms to disappear and part of his feet to bulge. Giving Snake Donkey Kong's moves makes Snake's body look stretched. Also, if one were to give Peach Kirby's moves her mouth is frozen in a stretched position, and will look like Edvard Munch's The Scream.