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 - $1.200
Lightly Played - $1.000
Moderately Played - $900
Heavily Played - $700
Damaged - $600
Near Mint Spanish - $1.200
Lightly Played Spanish - $1.000
Moderately Played Spanish - $900
Heavily Played Spanish - $700
Damaged Spanish - $600