They either cost zero mana (like Endurance), or the game ended when it entered the battlefield (Dauthi Voidwalker or Sanctifier en -Vec)
Theres better
However, Leafeon is useful compared to Serperior because you can pull energy to Celebi for free while using the energy you get by default each round on Leafeon
New Creature Types in Each Set/Block Lorwyn : Noggle Magic 2010 : Siren Zendikar : Ally, Surrakar, Eldrazi Scars of Mirrodin : Hippogriff, Germ, Praetor Innistrad : Werewolf Theros : God, Sable, Lamia, Nymph Khans of Tarkir : Naga Battle for Zendikar : Scion, Processor Shadows over Innistrad : Mole Kaladesh : Aetherborn, Pilot, Servo, Monkey Amonkhet : Jackal Ixalan : Trilobite, Dinosaur Battlebond : Azra Core Set 2019 : Egg War of the Spark : Army Commander 2019 : Sculpture Throne of Eldraine : Mouse, Peasant, Warlock, Noble Theros Beyond Death : Demigod, Tentacle Ikoria: Lair of Behemoths : Otter, Shark Core Set 2021 : Dog Kaldheim : Phyrexian Strixhaven: School of Mages : Inkling, Fractal Modern Horizons 2 : Ranger, Phyrexian creature type update Dungeons & Dragons: Adventures in the Forgotten Realms : Bard, Beholder, Gnoll, Halfling, Hamster Tiefling The Lost Caverns of Ixalan : Capybara, Llama, Snail Outlaws of Thunder Junction : Armadillo, Mount, Coyote, Porcupine, Possum, Varmint Bloomburrow : Skunk, Weasel Duskmourn: House of Horror : Glimmer, Toy, Final Fantasy : Moogle, Qu, Edge of Eternities: Drix, Lobster Spider-Man : Symbiote, Villain Universes Beyond: Echidna, Hero Some creature types have also been removed over the years
This means that sealed versions in good condition and cards with high grades are incredibly hard to come by
