"Mythical Beast" is an archetype of Spellcaster monsters that debuted in Structure Deck: Spellcaster's Judgment, with further members and support introduced in the Booster Pack Extreme Force. This archetype consists of "Mythical Beast Cerberus" and Pendulum Monsters based on it.
"Gazelle the King of Mythical Beasts" and "Chimera the Flying Mythical Beast" are not members of the archetype. However, because support for this archetype only works on Pendulum Monsters in the Pendulum Zone or Extra Deck, and Effect Monsters in the Main Deck, neither will be an issue.
The "Mythical Beast" archetype revolves around gaining Spell Counters and removing them to activate monster effects. The "Mythical Beast" monsters have monster effects that gain Spell Counters each time a Spell Card is activated. The lower-Level monsters gain 1 per Spell Card activation, while the higher-Level monsters gain 2 per activation. Their individual monster effects, which require removing Spell Counters to activate, include searching, recovery, card negates, and card removal.
Though the archetype consists primarily of Pendulum Monsters, it is very ineffective at performing Pendulum Summons. All "Mythical Beast" Pendulum Monsters have the same Pendulum Scale of 4, making it impossible to Pendulum Summon with just these cards, and complicating Pendulum Summons with other Pendulum Scales since 4 is a middling number. Their nature as Pendulum Monsters is instead meant to support their Spell Counter playstyle, as they can be activated in the Pendulum Zone as Spell Cards to generate Spell Counters for "Mythical Beasts" in the Monster Zones.
The Pendulum Effects of "Mythical Beast" Pendulum Monsters provide more searching, recovery, and a Spell/Trap removal. Their Pendulum Effects destroy the card itself as part of its effect, freeing up the Pendulum Zone for more activations. However, these Pendulum Effects can only be activated if the player controls no cards in the other Pendulum Zone.
The archetype synergizes well with the "Endymion" archetype. "Endymion" provides better Pendulum Scales for Pendulum Summoning, has a need for Spell Counters which the "Mythical Beast" archetype can rapidly generate, and also revolves around Spellcaster-Type Pendulum Monsters that can have Spell Counters placed on them, opening up more combo plays.