Building a proper five-color mana base that can reliably cast a creature like The First Sliver early in the game isn't always easy, so a little help in making those colors can really come in handy sometimes
They were a lot common when I was playing below Diamond
March of the Machine Phyrexia: All Will Be One The Brothers' War Dominaria United Streets of New Capenna Kamigawa: Neon Dynasty Innistrad: Crimson Vow Innistrad: Midnight Hunt Adventures in the Forgotten Realms Strixhaven Kaldheim Zendikar Rising Core 2021 Ikoria Lair of Behemoths Theros Beyond Death Throne of Eldraine Core Set 2020 War of the Spark Ravnica Allegiance Guilds of Ravnica Core Set 2019 Dominaria Rivals of Ixalan Ixalan Hour of Devastation Amonkhet Aether Revolt Kaladesh Eldritch Moon Shadows over Innistrad Oath of the Gatewatch Battle for Zendikar Magic Origins Dragons of Tarkir Fate Reforged Khans of Tarkir Magic 2015 Journey into Nyx Born of the Gods Theros Magic 2014 Dragon's Maze Gatecrash Return to Ravnica MTG Pioneer banlist But not every card from those sets is legal in Pioneer

Mirrym is one of blessed few Dragons who can make a case for being the Commander over The Ur-Dragon (though according to EDHREC, its not really close, around 2x as many decks as Miirym) and Klauth is one of the biggest swings you can do
And Order of the Sacred Torch can be a real annoyance, especially because the decks that care about it can have their removal countered by it
Pull rate from Regular Packs: 1.66% The estimated pull rate for Glaceon ex (Triumphant Light) from a Regular Pack is approximately 1.66%, meaning you might need to open around 61 Regular Packs to obtain it