Thundering Mightmare is a rare Creature — Horse Spirit card from Magic Online Promos (PRM), collector number 95413.
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."