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 - $1.100
Lightly Played - $900
Moderately Played - $800
Heavily Played - $700
Damaged - $600
Near Mint Spanish - $1.100
Lightly Played Spanish - $900
Moderately Played Spanish - $800
Heavily Played Spanish - $700
Damaged Spanish - $600