"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" monsters appear to be creatures merged with "Mythical Beast Jackal", which usually appears partially on the body of the creature. The transformation process is shown in the artwork of "Mythical Bestiary". For "Mythical Beast Medusa", "Jackal" is shown as the part of the monster's 'hair', while for "Mythical Beast Master Cerberus", it wears armor and holds a scepter whose designs are based on "Jackal".
Their effects revolve around gaining Spell Counters and using them to pay for effects.
Each Pendulum Monster that is a member of this archetype has a Pendulum Scale of 4 and has a Pendulum Effect that can only be activated if you control no cards in your other Pendulum Zone, and which destroys the card itself as part of its effect. These characteristics let them be played similarly to Normal Spell Cards to help build up Spell Counters on the "Mythical Beasts" in your Monster Zones.
Since they cannot set up a Pendulum Summon without outside support, in competitive gameplay this Deck has seen little "pure" builds, but other Pendulum Decks often use "Mythical Beast Master Cerberus" and "Mythical Beast Jackal King" as an engine. This is due to their easy Summoning for "Heavymetalfoes Electrumite", synergy with "Chronograph Sorcerer" and "Astrograph Sorcerer" (Forbidden in the TCG) and powerful negation/removal effects.
This is not to say the Deck is incapable of standing alone, however. With a sufficiently powerful Spell Counter/consistency engine like Spellbooks, the Deck can prove highly efficient at backing up the above engine to support their powerful effects. "Jackal" can be an effective way to Summon them without need for the Extra Monster Zone or Pendulum/Tribute Summoning. "Bashilisk" can recycle them without the loss in hand advantage or Deck thickening normally associated with this type of effect. "Medusa" can control the Battle Phase and revive off-archetype Spell Counter monsters. Finally, "Garuda" can remove problematic Spells/Traps offensively, or act as a amazing handtrap capable of essentially undoing an opponent's Normal Summon while bringing itself to the field for Link fodder/retrieval.
The archetype also has some good Spell support. "Beast Magic Attack" is a Quick-Play Spell that has 3 effects that cost a differing amount of Spell Counters from your field to use. The first, for 2 Spell Counters, returns a "Mythical Beast" from the field to the hand, useful to dodge removal effects (especially banishing) or recycle "Garuda". The second, for 4, Special Summons any face-up "Mythical Beast" from the Extra Deck, and adds 2 Spell Counters to it, essentially reviving any of your monsters to the Extra Monster Zone while immediately setting up their effects. The final effect, for 6, does the same for any Pendulum Monster without the Spell Counter placement, making it only useful on off-archetype monsters. "Mythical Institution" is a Continuous Spell that can help by accumulating 2 Spell Counters each time a "Mythical Beast" is destroyed (including by its own effect), and removing Spell Counters from your field to search/recover a "Mythical Beast" of the same Level as the amount removed. It can also protect itself from destruction by removing a Spell Counter, making it useful as a safe reserve of Spell Counters to power your effects.
- The Deck is heavily reliant on "Master Cerberus", not only as the main attacker, but also as a play-starter to build Spell Counters and search other cards.
- Cards which flip their monsters face-down, or banish their cards, remove valuable Spell Counters from the field while bypassing protection effects.
- While it is generally easy to build Spell Counters, it is often only at the cost of hand advantage if not opening optimally. Without Spell Counters, the Deck is defenceless.
- Being heavily Special-Summon reliant can be a crippling weakness against cards like "Vanity's Fiend".
- Being a Pendulum-based strategy in the Link Monster era makes this deck vulnerable to having all its monsters stuck in the Extra Deck, with few ways of recovering them, especially given how reliant on self-destruction they are.
- Cards like "Masked HERO Dark Law" can also be easily fatal for this Deck. Many of their combos rely on their monsters going to the Extra Deck for re-use, which is obviously impossible under such circumstances, and their self-destruction turns into a serious problem.