Thundering Mightmare 
Creature — Horse Spirit
$0.39 #10837 0.33 4.8 years
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."
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Cards similar to Thundering Mightmare expected a card that isn't here?
Reports become test cases the matcher is graded against before an update ships. Delvefall isn't a chatbot guessing, it compares the printed rules text of every card with plain math, and examples like yours decide which differences should count.
Taurean Mauler92%
$0.56↑#947↑0.3118.6 years↑
"Whenever an opponent casts a spell, you may put a +1/+1 counter on this creature."
Managorger Hydra90%
$0.99↑#997↑0.48↑11.1 years↑
"Whenever a player casts a spell, put a +1/+1 counter on this creature."
Ishai, Ojutai Dragonspeaker90%
$3.29↑#4668↑0.59↑9.8 years↑
"Whenever an opponent casts a spell, put a +1/+1 counter on this card."
+1 more matching line
Predatory Hunger89%
$0.65↑#107100.00↓28.2 years↑
"Whenever an opponent casts a creature spell, put a +1/+1 counter on enchanted creature."
Sunscorch Regent88%
$0.30↓#1686↑0.2911.4 years↑
"Whenever an opponent casts a spell, put a +1/+1 counter on this creature and you gain 1 life."
Medusa, Inhuman Queen84%
$0.27↓#18977↓0.00↓0.2 years↓
"Whenever a player casts a noncreature spell, put a +1/+1 counter on this card."
Forgotten Ancient82%
$0.38↓#390↑0.3723.3 years↑
"Whenever a player casts a spell, you may put a +1/+1 counter on this creature."
Mold Adder80%
$0.11↓#14542↓0.00↓17.1 years↑
"Whenever an opponent casts a blue or black spell, you may put a +1/+1 counter on this creature."
Citanul Druid80%
$8.63↑#28175↓0.53↑32.5 years↑
"Whenever an opponent casts an artifact spell, put a +1/+1 counter on this creature."
Witchstalker80%
$0.25↓#19122↓0.2713.1 years↑
"Whenever an opponent casts a blue or black spell during your turn, put a +1/+1 counter on this creature."
Titania's Chosen80%
$0.31↓#17141↓0.06↓27.9 years↑
"Whenever a player casts a green spell, put a +1/+1 counter on this creature."
Doom Weaver80%
$2.42↑#8354↑0.56↑4.8 years
"Soulbond"
Mirage Phalanx79%
$1.56↑#7002↑0.16↓4.8 years
"Soulbond"
Imperious Mindbreaker78%
$0.33↓#14457↓0.62↑4.8 years
"Soulbond"
Steelburr Champion78%
$0.28↓#6838↑0.242.1 years↓
"Whenever an opponent casts a noncreature spell, put a +1/+1 counter on this creature."
Dragonlair Spider74%
$0.47↑#8231↑0.10↓14.3 years↑
"Whenever an opponent casts a spell, create a 1/1 green Insect creature token."
Noosegraf Mob72%
$0.29↓#107930.3310.1 years↑
"Whenever a player casts a spell, remove a +1/+1 counter from this creature. If you do, create a 2/2 black Zombie creature token."
Stonewright71%
$0.06↓#27688↓0.00↓14.3 years↑
"Soulbond"
Stern Mentor70%
$0.04↓#28237↓0.2514.3 years↑
"Soulbond"
Pathbreaker Wurm70%
$0.06↓#21907↓0.20↓14.3 years↑
"Soulbond"