Thundering Mightmare is a rare Creature — Horse Spirit card from Crimson Vow Commander (VOC), collector number 75. Current market price: $0.65, TCGplayer market prices via Scryfall.
Soulbond (You may pair this creature with another unpaired creature when either enters. They remain paired for as long as you control both of them.) As long as Thundering Mightmare is paired with another creature, each of those creatures has "Whenever an opponent casts a spell, put a +1/+1 counter on this creature."