In the Pokémon as rare as Arceus. With such capability, would a Poké Ball be able to capture a human?
Poké Balls are specifically constructed for Pokémon capture. In both the Pokémon television series and the games, every attempt at catching a human with a Poké Ball has failed. The most recent instance of Poké Ball's failing to catch humans would be in the Pokémon: Sun and Moon Ultra Adventures episode "The Long Vault Home!". Ash and friends attempt to capture the Ultra Beast Stakataka, but it is elusive to their Poké Balls. Lillie has the idea in which everyone throws a Poké Ball at Stakataka in different directions, but Stakataka dodges them again and the Poké Balls are instead thrown to hit their faces, leading to the Pokémon still being uncaught.
However, something strange happens just two episodes before this in the Pokémon: Sun and Moon Ultra Adventures episode "Dummy, You Shrunk the Kids!". Visiting Professor Faba creates a machine that manipulates the sizes of objects and stores them in capsules, similar to the way that Poké Balls shrink down Pokémon to fit inside. The machine shrinks Ash, Lillie, and Sophocles into miniature sizes, and they are able to fit inside the Poké Ball-like capsules. This suggests that potentially humans and non-Pokémon animals could be captured within Poké Balls, albeit with extreme manipulation.
Pokémon Shows Humans Inside Poké Balls In Gen 5
There is a reality in which fans have seen humans being captured inside Poké Balls, which took place in Pokémon Black and White 2. Through the Pokéstar Studios near Virbank City, players could participate in the Timegate Traveler series. In this series, the future is ruled by Bug-type Pokémon that capture wild humans in "Human Balls" instead of the other way around. These "Human Balls" have no visual difference to Poké Balls. Within the entire Pokémon game franchise, this is the only known instance of humans being properly captured within a device that functions the exact same as Poké Balls without direct manipulation of the human being captured inside.
In the end, Poké Balls are devices still specifically constructed for Pokémon. The amount of manipulation that would have to take place for humans to be able to fit inside Poké Balls is dangerous and even Professor Faba's invention did not use the same technology that Poké Balls use. A Poké Ball thrown at a human with the intent to capture would only result in the person getting injured. There have been many Poké Balls thrown throughout the Pokémon franchise, and chances are, there are no other lifeforms besides Pokémon that can fit into the extraordinary devices.
