Good enough for Verazol, the Split Current and Hallar, the Firefletcher
Shadow Rider Calyrex is a bit of a "midrange" deck in the sense that it can do many different things during the game: it can KO opponents Pokmon with one big attack, it can negate prizes with healing, and it can slow down the game with different locks (Frillish and Wobbuffet) Why it wins Well, to put it simply, Calyrex can do many different things and all of them are very strong
Traditional Foil card and Art Card in every pack
Going by the username Xopher, he verified that he was behind the eBay listing, which put the card up for $50,000, or best offer
A lot of your cards will be able to crew this and so yeah, it is just an overly good card that fits neatly into this archetype
There are a few infinite loops that require a few specific cards but having one that can be used by so many different cards, this card definitely deserves its spot