Gruul Beastmaster 

Creature — Human Shaman
Riot (This creature enters with your choice of a +1/+1 counter or haste.)
Whenever this creature attacks, another target creature you control gets +X/+0 until end of turn, where X is this creature's power.
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Cards that do something similar expected a card that isn't here?
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Yotian Frontliner 97%
$0.22#20990
"Whenever this creature attacks, another target creature you control gets +1/+1 until end of turn."
Foot Elite 87%
$0.15#18022
"Whenever this creature attacks, another target creature you control gets +1/+0 and gains indestructible until end of turn."
Hardened Escort 87%
$0.12#15219
"Whenever this creature attacks, another target creature you control gets +1/+0 and gains indestructible until end of turn."
Ashroot Animist 87%
$0.19#9382
"Whenever this creature attacks, another target creature you control gains trample and gets +X/+X until end of turn, where X is this creature's power."
Oliphaunt 86%
$0.31#4130
"Whenever this creature attacks, another target creature you control gets +2/+0 and gains trample until end of turn."
Treetop Ambusher 85%
$0.07#26423
"Whenever this creature attacks, target creature you control gets +1/+1 until end of turn."
Wildwood Mentor 85%
$0.42#13429
"Whenever this creature attacks, another target attacking creature gets +X/+X until end of turn, where X is this creature's power."
Eel-Hounds 85%
$0.13#21042
"Whenever this creature attacks, another target creature you control gets +2/+2 and gains trample until end of turn."
Hardy Outlander 82%
$0.09#14423
"Commander creatures you own have "Whenever this creature attacks a player, if no opponent has more life than that player, another target creature you control gets +X/+X until end of turn, where X is this creature's power.""