Dragon Hatchling

Flying
{R}: Dragon Hatchling gets +1/+0 until end of turn.

Plains

Shivan Devastator

Flying, haste
Shivan Devastator enters the battlefield with X +1/+1 counters on it.

Frenzied Raider

Whenever you activate a boast ability, put a +1/+1 counter on Frenzied Raider.

Harnfel, Horn of Bounty

Discard a card: Exile the top two cards of your library. You may play those cards this turn.

Birgi, God of Storytelling

Whenever you cast a spell, add {R}. Until end of turn, you don't lose this mana as steps and phases end.
Creatures you control can boast twice during each of your turns rather than once.

Birgi, God of Storytelling // Harnfel, Horn of Bounty

Axgard Braggart

Boast — {1}{W}: Untap Axgard Braggart. Put a +1/+1 counter on it. (Activate only if this creature attacked this turn and only once each turn.)

Mountain

Usher of the Fallen

Boast — {1}{W}: Create a 1/1 white Human Warrior creature token. (Activate only if this creature attacked this turn and only once each turn.)

Battershield Warrior

Boast — {1}{W}: Creatures you control get +1/+1 until end of turn. (Activate only if this creature attacked this turn and only once each turn.)

Tuskeri Firewalker

Boast — {1}: Exile the top card of your library. You may play that card this turn. (Activate only if this creature attacked this turn and only once each turn.)

Reckless Barbarian

Sacrifice Reckless Barbarian: Add {R}{R}.

Arni Brokenbrow

Haste
Boast — {1}: You may change Arni Brokenbrow's base power to 1 plus the greatest power among other creatures you control until end of turn. (Activate only if this creature attacked this turn and only once each turn.)

Dragonkin Berserker

First strike
Boast abilities you activate cost {1} less to activate for each Dragon you control.
Boast — {4}{R}: Create a 5/5 red Dragon creature token with flying. (Activate only if this creature attacked this turn and only once each turn.)

Leyline Tyrant

Flying
You don't lose unspent red mana as steps and phases end.
When Leyline Tyrant dies, you may pay any amount of {R}. When you do, it deals that much damage to any target.

Fearless Liberator

Boast — {2}{R}: Create a 2/1 red Dwarf Berserker creature token. (Activate only if this creature attacked this turn and only once each turn.)