Spitemare

Whenever Spitemare is dealt damage, it deals that much damage to any target.

Wrathful Raptors

Trample
Whenever a Dinosaur you control is dealt damage, it deals that much damage to any target that isn't a Dinosaur.

Howlpack Avenger

Whenever a permanent you control is dealt damage, Howlpack Avenger deals that much damage to any target.
{1}{R}: Howlpack Avenger gets +2/+0 until end of turn.
Nightbound (If a player casts at least two spells during their own turn, it becomes day next turn.)

Ill-Tempered Loner

Whenever Ill-Tempered Loner is dealt damage, it deals that much damage to any target.
{1}{R}: Ill-Tempered Loner gets +2/+0 until end of turn.
Daybound (If a player casts no spells during their own turn, it becomes night next turn.)

Ill-Tempered Loner // Howlpack Avenger

Coalhauler Swine

Whenever Coalhauler Swine is dealt damage, it deals that much damage to each player.

Arcbond

Choose target creature. Whenever that creature is dealt damage this turn, it deals that much damage to each other creature and each player.

Truefire Captain

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

Flumph

Defender, flying
Whenever Flumph is dealt damage, you and target opponent each draw a card.

Boros Reckoner

Whenever Boros Reckoner is dealt damage, it deals that much damage to any target.
{R/W}: Boros Reckoner gains first strike until end of turn.

Brash Taunter

Indestructible
Whenever Brash Taunter is dealt damage, it deals that much damage to target opponent.
{2}{R}, {T}: Brash Taunter fights another target creature.

Wrathful Red Dragon

Flying
Whenever a Dragon you control is dealt damage, it deals that much damage to any target that isn't a Dragon.

Mogg Maniac

Whenever Mogg Maniac is dealt damage, it deals that much damage to target opponent or planeswalker.

Metropolis Reformer

Flying, vigilance
You have hexproof.
Whenever Metropolis Reformer is dealt damage, you gain that much life.

Expedited Inheritance

Whenever a creature is dealt damage, its controller may exile that many cards from the top of their library. They may play those cards until the end of their next turn.

Spiteful Sliver

Sliver creatures you control have "Whenever this creature is dealt damage, it deals that much damage to target player or planeswalker."

Donna Noble

Soulbond (You may pair this creature with another unpaired creature when either enters. They remain paired for as long as you control both of them.)
Whenever Donna or a creature it's paired with is dealt damage, Donna deals that much damage to target opponent.
Doctor's companion (You can have two commanders if the other is the Doctor.)

Fiendlash

Equipped creature gets +2/+0 and has reach.
Whenever equipped creature is dealt damage, it deals damage equal to its power to target player or planeswalker.
Equip {2}{R}

Raptor Hatchling

Enrage — Whenever Raptor Hatchling is dealt damage, create a 3/3 green Dinosaur creature token with trample.

Trapjaw Tyrant

Enrage — Whenever Trapjaw Tyrant is dealt damage, exile target creature an opponent controls until Trapjaw Tyrant leaves the battlefield.