Skanos Dragonheart 

Legendary Creature — Dragon Ranger
Whenever Skanos Dragonheart attacks, it gets +X/+X until end of turn, where X is the greatest power among other Dragons you control and Dragon cards in your graveyard.
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Cards that do something similar 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.
Edgar, Master Machinist 90%
$0.22#9751
"Whenever this card attacks, it gets +X/+0 until end of turn, where X is the greatest mana value among artifacts you control."
Tuya Bearclaw 88%
$0.19#10264
"Whenever this card attacks, it gets +X/+X until end of turn, where X is the greatest power among other creatures you control."
Targ Nar, Demon-Fang Gnoll 88%
$0.23#18419
"Whenever this card attacks, if you attacked with creatures with total power 6 or greater this combat, attacking creatures get +1/+0 until end of turn."
Moon-Boy, Dino Rider 86%
$0.95#26768
"Whenever this card attacks while you control a Dinosaur, this card gets +1/+1 until end of turn."
Jor Kadeen, First Goldwarden 86%
$0.29#9255
"Whenever this card attacks, it gets +X/+X until end of turn, where X is the number of equipped creatures you control. Then if this card's power is 4 or greater, draw a card."
Imaryll, Elfhame Elite 85%
#14218
"Whenever this card attacks, it gets +X/+X until end of turn, where X is the number of other Elves you control."
Pianna, Nomad Captain 83%
$0.15#21358
"Whenever this card attacks, attacking creatures get +1/+1 until end of turn."
Iroh, Firebending Instructor 83%
#16205
"Whenever this card attacks, attacking creatures get +1/+1 until end of turn."
Killmonger, Ruthless Usurper 82%
$0.31#18134
"Whenever this card attacks, he gets +1/+0 until end of turn for each artifact defending player controls."
Ruby, Daring Tracker 81%
$0.33#2171
"Whenever this card attacks while you control a creature with power 4 or greater, this card gets +2/+2 until end of turn."
Firebird, Blazing Ranger 81%
$0.34#18766
"Whenever this card attacks, other attacking creatures get +X/+0 until end of turn, where X is this card's power."
Butch DeLoria, Tunnel Snake 81%
$0.30#11274
"Whenever this card attacks, it gets +1/+1 until end of turn for each other Rogue and/or Snake you control."
Namor, Atlantean King 80%
$0.69#21886
"Whenever this card attacks a player who has more life than you, other creatures you control attacking that player get +2/+0 until end of turn."