From Bulbapedia, the community-driven Pokémon encyclopedia.
An outsider Pokémon is a class of Pokémon in the Pokémon games that is not originally from the same save file as the player's.
Once on the player
's game, Elyssa's Farfetch'd
is classed as an outsider Pokémon
A Pokémon is recognized as an outsider if its recorded data about its original Trainer does not match that of the player for the current game. In Generation II, this check only includes the Trainer ID number. In Generation III and IV, this check includes the original Trainer's name, Trainer ID number, and Secret ID.
When used in a battle, outsider Pokémon may disobey their Trainer if the Trainer does not have enough Gym Badges. This disobedience may range from using a different move, "loafing around," or even taking a nap. Canonically, Pokémon respect Trainers who have defeated Gym Leaders, and so, by earning more Badges, a Trainer will gradually earn the trust of an increasing variety of Pokémon. As a practical matter, this limitation also prevents high-leveled Pokémon from being traded from one game to easily beat another.
Outsider Pokémon also grow faster than native Pokémon. Outsider Pokémon gain 1.5 times the normal experience points for participating in battles. If the outsider Pokémon is also from another country (in Generations IV and V) or from a game in a different language (in Generation VI), it will gain 1.7 times the normal experience instead.
Outsider Pokémon also cannot be renamed by the Name Rater. Only a Pokémon's original Trainer is ever able to change its nickname.
Methods of obtaining outsider Pokémon
Outsider Pokémon can be obtained in a number of ways. If the player has multiple game paks and game systems, a Pokémon may be exchanged in a trade. He or she may also trade with a friend's game and (Generation IV onwards) via Global Trade Station. There are also a number of in-game trades available in every core series game, with a number of non-player characters willing to trade one of their Pokémon (which may or may not be otherwise obtainable) for one of the player's.
Sometimes non-player characters will gift the player with a Pokémon without requiring an exchange, but usually these Pokémon will not be treated as outsiders (having the same ID and OT name as the player). In some instances, however, they are, such as Webster's Spearow in Gold, Silver, Crystal, HeartGold, and SoulSilver and N's Pokémon in Black 2 and White 2.
Pokémon obtained from event distributions have predefined original Trainers, and so are outsider Pokémon.