From Bulbapedia, the community-driven Pokémon encyclopedia.
Rental Pokémon (Japanese: レンタルポケモン Rental Pokémon) are any Pokémon which a Trainer does not formally own and uses temporarily for a battle or competition. Using a rental Pokémon does not count as catching a Pokémon for filling up a Pokédex. They are most commonly used to test a Trainer's ability with using any Pokémon, even those they did not train and raise themselves.
In the games
Pokémon Stadium was the first game to make use of rental Pokémon. In this game, the different tournament levels have one of every Pokémon that can enter the tournament available for rental. Each Pokémon has a pre-determined move set for each tournament level. This provides the advantage of making it easier to compete in the different tournaments and to add statues of all Pokémon from the Generation I Pokédex to the Hall of Fame.
Rental Pokémon are used in this game in the same manner as the first Stadium.
Rental Pokémon can also be used in a Battle Now! feature, which semi-randomly generates teams of rental Pokémon for players to use. They are also used in the exams at Pokémon School.
Unlike their Stadium predecessors, these two games do not allow rental Pokémon for the competitions inside the games. However, they still have semi-random rental Pokémon teams available in their Battle Now! modes.
In the Battle Frontier, the Battle Factory is based around rental Pokémon. The player chooses a team of three out of six random rental Pokémon. Noland and Thorton, the Frontier Brains for the Battle Factories, also battle using random teams of rental Pokémon.
Like the Stadium games, rental Pokémon are available. However, not all kinds of Pokémon are available to rent at a time. Rental Passes are also the only kind of Battle Pass available for use at the Gateway Colosseum. Once a Rental Pass is earned, its Pokémon can be swapped between Rental Passes to build a team. Later on players can earn the ability to trade rental Pokémon with their opponents in Gateway Colosseum, and keep them after winning.
In the Pokémon World Tournament, the Rental Tournament and the Rental Master Tournament are, as their names suggest, based around rental Pokémon. The player chooses a team of three out of six random rental Pokémon to compete in these tournaments. Rental Pokémon can also be used while filming in Pokéstar Studios.
Players can scan QR codes of teams found on the Pokémon Global Link website to copy another players' team and use it as a Battle Box team. When Pokémon are copied in this way, shininess is not retained.
The Battle Tower has 5 rental teams you can use. These teams replace your regular party at the Battle Tower. All rental Pokémon have neutral natures.
In the anime
Ash using a rental Meowth in the anime
As part of the Pokémon League Admissions Exam, Ash and James battled an instructor using teams of three random rental Pokémon in The Ultimate Test. James picked a team of Pikachu, Charizard, and Ivysaur, but was disqualified after he sent out two of them at once. Ash picked a team of Weezing, Arbok, and Meowth. With this team, he lost to the examiner, but was able to defeat the Team Rocket trio.
The Battle Park in Johto, which appeared in One Trick Phony!, also uses rental Pokémon. James managed to defeat Ash in a battle using a rental Venusaur, but Jessie lost to him with a rental Charizard and Blastoise. These rental Pokémon are also trained to only obey orders while inside the park, and will start disobeying their renters if taken outside of the park limits, as demonstrated when Team Rocket tried to steal them.
Brock taking care of his rental Magnemite
The Pokémon Summer Academy, which Ash, his friends, and Jessie participated in during Pokémon the Series: Diamond and Pearl, included a couple of events involving rental Pokémon. The first event was a test of taking care of and battling with a random Pokémon. Ash got a Raichu, Brock got a Magnemite, Dawn got a Grimer, and Jessie got a Smoochum. After a day of caretaking and learning to know their Pokémon, and another day of battling, the students returned the rental Pokémon to the academy.
The final event of Pokémon Summer Academy also involved the use of rental Pokémon, this time as a part of a Pokémon Triathlon, with the first two of the triathlon's three legs involving riding a semi-random Pokémon suitable for the leg. For the first leg, the riding portion, Ash used a Spoink, Brock used an Onix, Dawn used a Dodrio, and Jessie used a Hippowdon. For the second leg, the water portion, Ash used a Mantyke, Brock used a Feraligatr, Dawn used a Sharpedo, and Jessie used a Magikarp.
In the manga
Rental Pokémon stolen by Guile Hideout in Pokémon Adventures
In the Pokémon Adventures manga
Multiple rental Pokémon appeared at the Battle Frontier in the Emerald chapter. Besides the Battle Factory, rental Pokémon are also used in other facilities (excluding the Battle Dome due to its tournament-style rules) by computer-controlled virtual Trainers working as the challengers' opponents.
During his Battle Factory challenge, Emerald rented a Sceptile, which he noticed to be on a higher level than the other rentals he used, making him realize that it wasn't a rental Pokémon to begin with, and had been snuck into the facility by someone. This prompted him to take Sceptile with him. Ruby later revealed to him that Sceptile was the very same Treecko that Emerald had once been supposed to receive from Professor Birch.
All the rental Pokémon from the Battle Factory were later stolen by the villainous Guile Hideout, using them against those opposing him while he was hunting down Jirachi. Once he successfully caught Jirachi, he unleashed the rental Pokémon upon the members of the press present during the Battle Frontier opening ceremony. He also used the rental Pokémon inside the Battle Tower to attack the Pokédex holders until all the rampaging Pokémon were calmed down by Emerald's mud pellets.
Multiple rental Pokémon appeared at the Battle Factory in the Platinum chapter. The nature of the Battle Factory challenge allowed Platinum to send her own Pokémon off to help Buck and Looker at Stark Mountain while she challenged the facility. Later, during the same chapter, she also faced Argenta's rental Dragonite at the Battle Hall.
In other languages