Leonardo, Big Brother

Sneak {W} (You may cast this spell for {W} if you also return an unblocked attacker you control to hand during the declare blockers step. He enters tapped and attacking.)
Leonardo gets +1/+0 for each other creature you control.

Plains

Island

Mouser Mark III

This creature can't attack unless you control another artifact.

Nobody

When this creature enters, return up to one other target artifact you control to its owner's hand. Scry 1. (Look at the top card of your library. You may put that card on the bottom.)

Donatello, Way with Machines

Flying
Whenever an artifact you control enters, put a +1/+1 counter on Donatello.

April O'Neil, Kunoichi Trainee

When April O'Neil enters, scry 2. (Look at the top two cards of your library, then put any number of them on the bottom and the rest on top in any order.)
April O'Neil can't be blocked by creatures with power 3 or greater.

East Wind Avatar

Flying, vigilance
Alliance — Whenever another creature you control enters, this creature gets +1/+0 until end of turn.

Don & Leo, Problem Solvers

Vigilance
At the beginning of your end step, exile up to one target artifact you control and up to one target creature you control. Then return them to the battlefield under their owners' control.

Hamato Guardian Stance

Target creature gets +1/+3 and gains flying until end of turn. Scry 1. (Look at the top card of your library. You may put that card on the bottom.)

Triceraton Commander

Flying
Whenever this creature attacks, Dinosaurs you control other than this creature get +1/+1 and gain flying until end of turn.
When this creature enters, create X 2/2 white Dinosaur Soldier creature tokens.

Return to the Sewers

Target creature's owner puts it on their choice of the top or bottom of their library. You create a Mutagen token. (It's an artifact with "{1}, {T}, Sacrifice this token: Put a +1/+1 counter on target creature. Activate only as a sorcery.")

Quintessential Katana

Equipped creature gets +1/+1 and has "Whenever this creature deals combat damage, untap it and you gain 2 life."
Whenever a Ninja you control enters, you may attach this Equipment to it.
Equip {2} ({2}: Attach to target creature you control. Equip only as a sorcery.)

Utrom Scientists

When this creature enters, tap up to one target creature and put a stun counter on it. (If a permanent with a stun counter would become untapped, remove one from it instead.)

Leonardo, Cutting Edge

Sneak {W} (You may cast this spell for {W} if you also return an unblocked attacker you control to hand during the declare blockers step. He enters tapped and attacking.)
Lifelink
Whenever you gain life, put a +1/+1 counter on Leonardo.

Fugitive Droid

This creature can't be blocked if an artifact entered the battlefield under your control this turn.
{U}, Sacrifice this creature: Counter target spell that targets an artifact or creature you control.

Sewer-veillance Cam

Flash
When this artifact enters or leaves the battlefield, you may tap or untap target creature.
{3}{U}, Sacrifice this artifact: Draw two cards.