The Boss Yo-kai (Japanese: ボス妖怪,Bosu Yōkai) are powerful enemy Yo-kai which the player cannot befriend or recruit, and are typically stronger than most Yo-kai the player has met up to that point.
Most of the chapters in the Yo-kai Watch video games end with a battle with one of these bosses, and they are registered in the last few pages of the Yo-kai Medallium, just after the Legend Pages; however, they are not assigned any true Rank or Tribe, with the Boss symbol being in place of a Tribe's own.
Some Boss Yo-kai become befriendable in future games following their debut, giving them a Tribe and a Rank as well as a smaller, more compact form, which is not considered a Boss Yo-kai. Most bosses belong to the Shady tribe, Eerie tribe, Slippery tribe, Brave tribe, or Tough tribe due to them being sneaky with evil or bad intentions or their high defensive and offensive powers. Also, almost all bosses have a negative Inspiritment effect.
With the release of Yo-kai Watch Blasters and the release of the DX Yo-kai Watch U Prototype, the Yo-kai bosses featured in the game were given B-medals. Like the other B-medals, their medals are black-framed, and their artwork features the Bosses in action.
However, there are some differences. Their medals don't play the traditional summoning music of any of the Yo-kai Tribes when inserted into the DX Yo-kai Watch U, but merely an ominous electric guitar. In addition, Boss Yo-kai Medals do not have any tribe-affiliated insignias on them; rather, they have a slash going through them.
In Yo-kai Watch 3, the bosses are no longer given numbers that continue from the befriendable/normal Yo-kai Medallium number. Instead, the Boss Yo-kai numbering restarts the numbering system, and adds a B for Boss in prefix form to the number.
Despite befriendable bosses like Slimamander or SV Snaggerjag gaining friendlier-looking forms, their medals still showcase their boss forms, indicating that all Boss Yo-kai have regular Yo-kai Medals that are simply unseen and never granted to the player.