The Yo-kai Tribes (Japanese: 妖怪の種族 Yōkai no Shuzoku) are different groups that every species of Yo-kai is divided into. In battle, two or more adjacent Yo-kai in battle that hail from the same tribe receive a special Tribe Unity bonus, giving those Yo-kai a special boost to a stat that is best exemplified by that tribe.
In the games
The first Yo-kai Watch game introduced 8 tribes, with a 9th group called the Legendary Yo-kai, though they are not a separate tribe in themselves, as those Yo-kai still belong to one of the eight Yo-kai tribes.
In Yo-kai Watch 2: Bony Spirits and Fleshy Souls, an official 9th Yo-kai tribe, the Wicked tribe was introduced, though it wasn't until Psychic Specter that the Wicked tribe Yo-kai could be usable in battle, and even then, only the Wicked Executives and the Wicked-controlled Classic Yo-kai had any Yo-kai Medals to give. Also, the Wicked Tribe is unique in that there is no Tribe Unity bonus.
In Yo-kai Watch 3, the existence of a eleventh Yo-kai tribe, the Wandroid tribe, was leaked via CoroCoro magazine following the finalist results of fans submitting their designs with the first and only member being Sighborg Y.
In Yo-kai Watch Shadowside, almost all of the old tribes have been replaced with 10 new tribes (Goriki, Onnen, Mononoke, Tsukumono, Uwanosora, Omamori, Mikakunin, Oni, Izana and Mikado), with only the Wicked tribe remaining the same.