They were first introduced with 2021's Evolving Skies
Unfortunately, that power is locked behind one of the more obnoxious transform conditions in the set
According to Magic: The Gatherings Gatherer card database, at the time of writing there are more than 2,000 legendary creatures in the game

1 2 3 4 5 6 7 A ------------- B ------------- C ------------- D ------------- E ------------- F ------------- G ------------- Go to these districts for the appropriate card: Dust Tornado (Mai, E7) Carat Idol (Yugi, D1) Shape Snatch (Bakura, A7) Rigras Leever (Joey, E7) Time Eater (Mako, E6) Servant of Catabolism (Tea, E5) Dharma Cannon (Grandpa, C4) Eternal Rest (Necromancer, A1) Eradicating Aerosol (Weevil, D7) Cold Wave (Rex Raptor, A7) Exile of the Wicked (Panik/Darkness Ruler, B6) Jowls of Demise (Isthar, D4) Novox Prayer (Tristan, D5) Electromagnetic Bagworm (Pegasus, A4) Strain Storm (Bandit Keith, C4) Souleater (Shadi, D4) Catapult Turtle (Labyrinth Ruler, A7) Warrior Elimination (King, B4) Moisture Creature (Kaiba, D4) Unlock Reincarnate By surrendering five times or getting five wins, you will be able to reincarnate a card in your chest to allow you to build your deck