Shapeshifter's Marrow 
Enchantment
$0.51 #26125 0.07 19.5 years
At the beginning of each opponent's upkeep, that player reveals the top card of their library. If it's a creature card, the player puts the card into their graveyard and this enchantment becomes a copy of that card. (If it does, it loses this ability.)
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Cards similar to Shapeshifter's Marrow 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.
Vesuvan Drifter80%
$0.29↓#10558↑0.073.4 years↓
"At the beginning of each combat, you may reveal the top card of your library. If you reveal a creature card this way, this creature becomes a copy of that card until end of turn, except it has flying."
Shadow Kin78%
$0.69↑#8120↑0.26↑5.1 years↓
"At the beginning of your upkeep, each player mills three cards. You may exile a creature card from among the cards milled this way. If you do, this creature becomes a copy of that card, except it has this ability."
Cryptoplasm74%
$0.64↑#12938↑0.44↑15.7 years↓
"At the beginning of your upkeep, you may have this creature become a copy of another target creature, except it has this ability."
Cemetery Puca72%
$2.76↑#19936↑0.0718.5 years
"Whenever a creature dies, you may pay 
. If you do, this creature becomes a copy of that creature, except it has this ability."
. If you do, this creature becomes a copy of that creature, except it has this ability."
Deceiver of Form70%
$0.46↓#10245↑0.37↑10.7 years↓
"At the beginning of combat on your turn, reveal the top card of your library. If a creature card is revealed this way, you may have creatures you control other than this creature become copies of that card until end of turn. You may put that card on the bottom of your library."