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.