"Phantom Beast" (幻獣Genjū) is an archetype of mostly Beast-Warrior and Machinemonsters.
"Gazelle the King of Mythical Beasts" and "Chimera the Flying Mythical Beast" are also "Phantom Beast" cards because they have 「幻獣」 in their Japanese names. In the TCG, "Gazelle the King of Mythical Beasts" and "Chimera the Flying Mythical Beast" each have an archetype condition that treats it as a "Phantom Beast" monster. Due to no "Phantom Beast" support cards being printed since the archetype condition was added to "Gazelle the King of Mythical Beasts", it is always specifically included in effects that specify "Phantom Beast" monsters.
The archetype also includes a sub-archetype, the "Mecha Phantom Beasts". The Chimera series also shares many of its members with this archetype.
Playing style
They are based around aggressive beatdown tactics, gaining advantages when sent to/while in the Graveyard, or inflicting extra damage when battling.
Banishers like "Disappear", "Soul Release" and "Kycoo the Ghost Destroyer" can take "Cross Wing", "Thunder Pegasus" and/or "Chimaera"'s Fusion Material Monsters out of the Graveyard and hence nullify a motley of benefits that "Phantom Beasts" can't otherwise get much of from other support cards. This can be contested with "Imperial Iron Wall".
"Tualatin" is a perfect mismatch against standard "Phantom Beast" Decks as this card can keep them off the field wholly, an Archetype that is heftily needy on field presence and offensive stratagems.