Avatar: The Last Airbender Eternal

Legendary Creature — Human Noble Ally

Haste
Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
At the beginning of combat on your turn, each creature you control with a counter on it gains firebending 2 until end of turn. (Whenever it attacks, add {R}{R}. This mana lasts until end of combat.)

Iroh, Dragon of the West

Tarkir: Dragonstorm Commander

Goblin

Creature — Goblin Soldier

Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
At the beginning of combat on your turn, create a 1/1 red Goblin creature token. That token gains haste until end of turn and attacks this combat if able.

Legion Warboss

Foundations Jumpstart

Creature — Angel

Flying
Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
When this creature dies, put a +1/+1 counter on each white creature you control.

Light of the Legion

Creature — Human Knight

Flying
Vigilance (Attacking doesn't cause this creature to tap.)
Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)

Parhelion Patrol

Creature — Human Soldier

First strike (This creature deals combat damage before creatures without first strike.)
Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)

Sunhome Stalwart

Modern Horizons 3

Enchantment Creature — Unicorn

Bestow {3}{W} (If you cast this card for its bestow cost, it's an Aura spell with enchant creature. It becomes a creature again if it's not attached.)
Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
Enchanted creature gets +2/+2 and has mentor.

Nyxborn Unicorn

Ravnica: Clue Edition

Creature — Minotaur Soldier

Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
Whenever you attack a player, target creature that's attacking that player gains first strike until end of turn.

Ordruun Mentor

Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
This creature can't attack or block alone.

Wojek Bodyguard

Ravnica Remastered

Legendary Creature — Angel

Flying
Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
At the beginning of combat on your turn, choose up to one target creature you control. Until end of turn, that creature gets +2/+0, gains trample if it's red, and gains vigilance if it's white.

Aurelia, Exemplar of Justice

Legendary Creature — Human Soldier

Haste
Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
Prevent all noncombat damage that would be dealt to other creatures you control.
{R}{W}: Tajic gains first strike until end of turn.

Tajic, Legion's Edge

Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
Whenever this creature is dealt damage, it deals that much damage to target player.

Truefire Captain

The Lost Caverns of Ixalan Commander

Creature — Merfolk Shaman

Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
Whenever a creature you control with a +1/+1 counter on it dies, draw a card.

Tributary Instructor

Doctor Who

Legendary Creature — Human Soldier Advisor

Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
Creatures you control have "Whenever one or more counters are put on this creature for the first time each turn, draw a card."

Danny Pink

Unknown Event

Legendary Creature — Dragon

Flying
Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
Whenever one or more +1/+1 counters are put on a creature you control, exile the top card of your library. Until end of turn, you may play that card.

The Powerful Dragon

Unfinity Sticker Sheets

Stickers

{TK}{TK} — Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
{TK}{TK}{TK} — When this permanent leaves the battlefield, you may destroy target creature with power 4 or greater.
{TK}{TK} — 2/3
{TK}{TK}{TK}{TK} — 8/4

Goblin Coward Parade

Double Masters 2022

Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
{1}{R}: This creature gets +2/+0 until end of turn.

Goblin Banneret

Commander Legends: Battle for Baldur's Gate

Inkling

Legendary Creature — Vampire Wizard

Flying
Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
Whenever a nontoken creature you control dies, if it had counters on it, create X tapped 2/1 white and black Inkling creature tokens with flying, where X is the number of counters it had on it.

Felisa, Fang of Silverquill

The List

Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)

Blade Instructor

Haste
Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)

Barging Sergeant

Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
{2}{R}{W}: This creature gets +1/+1 until end of turn.