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 - $0.30
Lightly Played - $0.20
Moderately Played - $0.20
Heavily Played - $0.20
Damaged - $0.20
Near Mint Spanish - $0.30
Lightly Played Spanish - $0.20
Moderately Played Spanish - $0.20
Heavily Played Spanish - $0.20
Damaged Spanish - $0.20