Cheapest priced copy (Crimson Vow Commander): $0.49 at Card Kingdom, near mint, their price list as of .
Thundering Mightmare 
Creature — Horse Spirit · 3/3 · Rare · Crimson Vow Commander · #75
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."
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
2 printings · $0.49 to $0.69 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Crimson Vow Commander | 37 | Rare | $0.49 | — |
| Crimson Vow Commander | 75 | Rare | $0.69 | — |