"Mystic Baby Dragon" and "Mystic Dragon". Artwork is from "Mystic Revolution".
The cards focus on Summoning stronger versions of themselves during the End Phase of a turn that they are destroyed by battle. Only four cards in this series, "Egg", "Baby Dragon", "Dragon" and "Revolution" have been seen. However, the lore of "Revolution" reveals that "Mystic Baby Magician" and "Mystic Baby Knight" also exist, and have upgraded forms (that are unnamed). "Mystic Dragon", the only "adult" form shown, has the current highest ATK of all Level 8 Monster Cards.
Similar to the "Chrysalis" monsters, the "Mystic" cards focus on playing weak monsters and using their effects to bring out stronger versions of themselves. The "Mystic" cards do this during the End Phase whenever one of them is destroyed by battle. This allows for the quick Summoning of powerful monsters, such as "Mystic Dragon".