Ballynock Trapper 

Creature — Kithkin Soldier
{T}: Tap target creature.
Whenever you cast a white spell, you may untap this creature.
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Zhalfirin Decoy 98%
$0.17#28489
"{T}: Tap target creature. Activate only if you had a creature enter the battlefield under your control this turn."
Sand Squid 97%
$0.61#25209
"{T}: Tap target creature. That creature doesn't untap during its controller's untap step for as long as this creature remains tapped."
+1 more matching line
Merrow Bonegnawer 88%
$0.24#16531
"Whenever you cast a black spell, you may untap this creature."
+1 more matching line
Leonin Battlemage 88%
$0.21#25413
"Whenever you cast a spell, you may untap this creature."
+1 more matching line
Merrow Levitator 86%
$0.26#25105
"Whenever you cast a blue spell, you may untap this creature."
+1 more matching line
Gustcloak Cavalier 84%
$0.11#27526
"Whenever this creature becomes blocked, you may untap this creature and remove it from combat."
Whip Vine 84%
$0.12#27095
"You may choose not to untap this creature during your untap step."
+1 more matching line
Mana Leech 84%
$0.20#29836
"You may choose not to untap this creature during your untap step."
+1 more matching line
Mole Worms 84%
$0.19#29646
"You may choose not to untap this creature during your untap step."
+1 more matching line
Phyrexian Gremlins 84%
$1.03#29985
"You may choose not to untap this creature during your untap step."
+1 more matching line
Preacher 84%
#22058
"You may choose not to untap this creature during your untap step."
+1 more matching line
Callous Oppressor 84%
$1.72#15408
"You may choose not to untap this creature during your untap step."
+1 more matching line
Rust Tick 84%
$0.20#25862
"You may choose not to untap this creature during your untap step."
+1 more matching line