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.”
Near Mint - $400
Lightly Played - $300
Moderately Played - $300
Heavily Played - $300
Damaged - $200
Near Mint Spanish - $400
Lightly Played Spanish - $300
Moderately Played Spanish - $300
Heavily Played Spanish - $300
Damaged Spanish - $200