"Yokai" (妖怪 Yōkai) is a group of Monster Cards originally from the Yu-Gi-Oh! GX manga. They were used by Bastion Misawa and are based on Yōkai creatures from Japanese folklore.
Their effects usually activate when they are Special Summoned from their owner's Graveyard.
It is unclear how Yokai Monsters are identified in the manga. The Japanese text of "Haunted Shrine" indicates that they are a Type, like Warrior, Dragon etc. The Japanese text of "Long-Tailed Black Horse" indicates that they are an archetype like "Archfiend", "Elemental HERO" etc., though no Yokai Monsters actually have the specified string "妖怪" in their Japanese names. The Japanese text of "Phantasmagoria" indicates they are a type of Monster Card like Toon, Tuner etc. For the purposes of this page, they are categorized as an archetype. In the real world OCG and TCG, Yokai Monsters are not directly connected in a gameplay sense, but are all EARTH Zombie-Type monsters, except for "Goka, the Pyre of Malice" which is a FIRE Zombie-Type monster.