As insane as Urshifu was with Unseen Fist, Game Freak decided to top themselves in the Crown Tundra. After escorting the amnesiac Calyrex around the tundra you are rewarded with two of the outright strongest Pokémon in the game (excluding Zacian). With both Rider Forms having Calyrex's decent support movepool alongside utterly busted STABs and Moxie for their STAB of choice, Calyrex's rider forms reign over both singles and doubles.
Riderless Calyrex on the other hand... 100 HP and straight 80s across the board otherwise is extremely unimpressive and with the utterly weakness laden Grass Psychic typing, many, many other Pokémon can do what Calyrex can do but better.
The Frozen King
Ability: As-One (Chilling Neigh + Unnerve)
Item: Weakness Policy
Nature: Brave (+ATK, -SPE)
EVs: 252 HP / 252 ATK / 4 SDEF
Doubles Calyrex is just insane. Trick Room is very potent Speed control in doubles formats, punishing Max Airstream, Tailwind, and most weather sweepers. And with its pitiful Speed, Ice Rider easily outslows the competition. Glacial Lance is a fantastic Spread STAB move that most physical Ice types would kill for. Thanks to being a Spread Ice move, it can easily pick up KOs from the many Flying, Dragon, Ground and Rillaboom types running around, easily getting one or even two Chilling Neigh boosts enabling Calyrex to quickly snowball out of control. High Horsepower covers most Fire and Steel types that resist Glacial Lance. Heavy Slam is a niche pick, punishing opposing Ice Riders and Xerneas, but its true value comes when Dynamaxed. Max Quake and Max Steelspike enable Calyrex to scale defensively, making it an immovable bulwark if it accrues enough boosts while not losing out on offense thanks to Chilling Neigh. To further escalate matters, thanks to its insane natural bulk and myriad of weaknesses, it is very easy for Ice Rider to get a Weakness Policy boost even outside of Dynamax, enabling Calyrex to ignore an Intimidate or two and quickly escalate to an unmanageable point.
If not running Heavy Slam, Protect is preferred thanks to the ability to scout, stall and gather information at minimal risk.
Helping Hand -If out of Trick Room and you know Calyrex is going to be KO'd, have it support its partner, enabling it to go down swinging.