Bounding Felidar 

Creature — Cat Beast Mount
Whenever this creature attacks while saddled, put a +1/+1 counter on each other creature you control. You gain 1 life for each of those creatures.
Saddle 2 (Tap any number of other creatures you control with total power 2 or more: This Mount becomes saddled until end of turn. Saddle only as a sorcery.)
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Cards that do something similar expected a card that isn't here?
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Bellowing Aegisaur 89%
$0.30#7788
"Whenever this creature is dealt damage, put a +1/+1 counter on each other creature you control."
Thunderwolf Cavalry 87%
$0.36#15063
"Whenever this creature deals combat damage to a player, put a +1/+1 counter on each other creature you control."
Generous Pup 87%
$5.32#5798
"Whenever one or more +1/+1 counters are put on this creature, put a +1/+1 counter on each other creature you control. This ability triggers only once each turn."
Wild Beastmaster 85%
$0.35#8287
"Whenever this creature attacks, each other creature you control gets +X/+X until end of turn, where X is this creature's power."
Soltari Champion 85%
#17048
"Whenever this creature attacks, other creatures you control get +1/+1 until end of turn."
Bolt Hound 85%
$0.14#13083
"Whenever this creature attacks, other creatures you control get +1/+0 until end of turn."
Kalonian Hydra 84%
$5.82#1096
"Whenever this creature attacks, double the number of +1/+1 counters on each creature you control."
Renegade Krasis 84%
$0.31#14686
"Whenever this creature evolves, put a +1/+1 counter on each other creature you control with a +1/+1 counter on it."
High Sentinels of Arashin 83%
$0.21#14978
"This creature gets +1/+1 for each other creature you control with a +1/+1 counter on it."
Fangren Firstborn 83%
$2.46#15366
"Whenever this creature attacks, put a +1/+1 counter on each attacking creature."
Titans' Vanguard 83%
$0.16#8020
"When you cast this spell and whenever this creature attacks, put a +1/+1 counter on each colorless creature you control."
Consul's Lieutenant 82%
$0.18#21483
"Whenever this creature attacks, if it's renowned, other attacking creatures you control get +1/+1 until end of turn."
Sigil of Valor 82%
$0.18#8438
"Whenever equipped creature attacks alone, it gets +1/+1 until end of turn for each other creature you control."
+1 more matching line
Oran-Rief Ooze 82%
$0.26#8026
"Whenever this creature attacks, put a +1/+1 counter on each attacking creature with a +1/+1 counter on it."
Rubblebelt Raiders 82%
$0.13#17880
"Whenever this creature attacks, put a +1/+1 counter on it for each attacking creature you control."
Finneas, Ace Archer 81%
$0.32#4373
"Whenever this card attacks, put a +1/+1 counter on each other creature you control that's a token or a Rabbit. Then if creatures you control have total power 10 or greater, draw a card."
Grotag Bug-Catcher 81%
$0.12#24173
"Whenever this creature attacks, it gets +1/+0 until end of turn for each creature in your party."
Invasion of New Capenna // Holy Frazzle-Cannon 81%
$0.21#9308
"Whenever equipped creature attacks, put a +1/+1 counter on that creature and each other creature you control that shares a creature type with it."
+1 more matching line
Rampaging Classmate 80%
$0.27#25040
"Whenever this creature attacks, it gets +1/+0 until end of turn for each other attacking creature."