Boreal Outrider is a uncommon Snow Creature — Elf Warrior card from Kaldheim (KHM), collector number 163. Current market price: $0.29 (foil $0.26), TCGplayer market prices via Scryfall.
Whenever you cast a creature spell, if {S} of any of that spell's colors was spent to cast it, that creature enters with an additional +1/+1 counter on it. ({S} is mana from a snow source.)