Have

Aven Mindcensor

Flash
Flying
If an opponent would search a library, that player searches the top four cards of that library instead.

Momentary Blink

Exile target creature you control, then return it to the battlefield under its owner's control.
Flashback {3}{U} (You may cast this card from your graveyard for its flashback cost. Then exile it.)

Don't Have

Containment Priest

Flash
If a nontoken creature would enter and it wasn't cast, exile it instead.

Selfless Squire

Flash
When Selfless Squire enters, prevent all damage that would be dealt to you this turn.
Whenever damage that would be dealt to you is prevented, put that many +1/+1 counters on Selfless Squire.

Quickling

Flash (You may cast this spell any time you could cast an instant.)
Flying
When Quickling enters, sacrifice it unless you return another creature you control to its owner's hand.

Guardian of Faith

Flash
Vigilance
When Guardian of Faith enters, any number of other target creatures you control phase out. (Treat them and anything attached to them as though they don't exist until their controller's next turn.)

Stonecloaker

Flash
Flying
When Stonecloaker enters, return a creature you control to its owner's hand.
When Stonecloaker enters, exile target card from a graveyard.

Mizzium Meddler

Flash (You may cast this spell any time you could cast an instant.)
When Mizzium Meddler enters, you may change a target of target spell or ability to Mizzium Meddler.

Portal Mage

Flash
When Portal Mage enters during the declare attackers step, you may reselect which player or permanent target attacking creature is attacking. (It can't attack its controller or their permanents.)

Pestermite

Flash
Flying
When Pestermite enters, you may tap or untap target permanent.

Githzerai Monk

Flash
Flying
Psychic Defense — When Githzerai Monk enters, tap all creatures you don't control.

Jeskai Barricade

Flash (You may cast this spell any time you could cast an instant.)
Defender
When Jeskai Barricade enters, you may return another target creature you control to its owner's hand.

Brineborn Cutthroat

Flash (You may cast this spell any time you could cast an instant.)
Whenever you cast a spell during an opponent's turn, put a +1/+1 counter on this creature.

Sea-Dasher Octopus

Mutate {1}{U} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Flash
Whenever this creature deals combat damage to a player, draw a card.

Cathar Commando

Flash (You may cast this spell any time you could cast an instant.)
{1}, Sacrifice this creature: Destroy target artifact or enchantment.

Theoretical Duplication

Whenever a nontoken creature an opponent controls enters this turn, create a token that's a copy of that creature.

Wavebreak Hippocamp

Whenever you cast your first spell during each opponent's turn, draw a card.

Saltskitter

Whenever another creature enters, exile Saltskitter. Return Saltskitter to the battlefield under its owner's control at the beginning of the next end step.

Aboleth Spawn

Flash
Ward {2}
Probing Telepathy — Whenever a creature entering under an opponent's control causes a triggered ability of that creature to trigger, you may copy that ability. You may choose new targets for the copy.

Whitemane Lion

Flash
When Whitemane Lion enters, return a creature you control to its owner's hand.

Planar Incision

Exile target artifact or creature, then return it to the battlefield under its owner's control with a +1/+1 counter on it.

Hullbreaker Horror

Flash
This spell can't be countered.
Whenever you cast a spell, choose up to one —
• Return target spell you don't control to its owner's hand.
• Return target nonland permanent to its owner's hand.

Leyline of Anticipation

If Leyline of Anticipation is in your opening hand, you may begin the game with it on the battlefield.
You may cast spells as though they had flash.

Familiar's Ruse

As an additional cost to cast this spell, return a creature you control to its owner's hand.
Counter target spell.

Village Bell-Ringer

Flash (You may cast this spell any time you could cast an instant.)
When Village Bell-Ringer enters, untap all creatures you control.

As Foretold

At the beginning of your upkeep, put a time counter on As Foretold.
Once each turn, you may pay {0} rather than pay the mana cost for a spell you cast with mana value X or less, where X is the number of time counters on As Foretold.

Windshaper Planetar

Flash
Flying
When Windshaper Planetar enters during the declare attackers step, for each attacking creature, you may reselect which player or permanent that creature is attacking. (It can't attack its controller or their permanents.)

Deputy of Acquittals

Flash (You may cast this spell any time you could cast an instant.)
When Deputy of Acquittals enters, you may return another target creature you control to its owner's hand.

Crafty Cutpurse

Flash
When Crafty Cutpurse enters, each token that would be created under an opponent's control this turn is created under your control instead.

Ephara, God of the Polis

Indestructible
As long as your devotion to white and blue is less than seven, Ephara isn't a creature.
At the beginning of each upkeep, if you had another creature enter the battlefield under your control last turn, draw a card.

Tidespout Tyrant

Flying
Whenever you cast a spell, return target permanent to its owner's hand.

Rescuer Chwinga

Flash
Natural Shelter — When Rescuer Chwinga enters, you may return another permanent you control to its owner's hand.