Johan

At the beginning of combat on your turn, you may have Johan gain "Johan can't attack" until end of combat. If you do, attacking doesn't cause creatures you control to tap this combat if Johan is untapped.

Devoted Crop-Mate

You may exert Devoted Crop-Mate as it attacks. When you do, return target creature card with mana value 2 or less from your graveyard to the battlefield. (An exerted creature won't untap during your next untap step.)

Themberchaud

Trample
When Themberchaud enters, he deals X damage to each other creature without flying and each player, where X is the number of Mountains you control.
You may exert Themberchaud as he attacks. When you do, he gains flying until end of turn. (An exerted creature won't untap during your next untap step.)

Rohirrim Chargers

You may exert Rohirrim Chargers as it attacks. (It won't untap during your next untap step.)
Whenever you exert a creature, reveal cards from the top of your library until you reveal an Equipment card. Put that card onto the battlefield attached to that creature, then put the rest on the bottom of your library in a random order.

Anger

Haste
As long as Anger is in your graveyard and you control a Mountain, creatures you control have haste.

Glory-Bound Initiate

You may exert Glory-Bound Initiate as it attacks. When you do, it gets +1/+3 and gains lifelink until end of turn. (An exerted creature won't untap during your next untap step.)

Trueheart Twins

You may exert Trueheart Twins as it attacks. (It won't untap during your next untap step.)
Whenever you exert a creature, creatures you control get +1/+0 until end of turn.

Watchful Naga

You may exert Watchful Naga as it attacks. When you do, draw a card. (An exerted creature won't untap during your next untap step.)

Champion of Rhonas

You may exert Champion of Rhonas as it attacks. When you do, you may put a creature card from your hand onto the battlefield. (An exerted creature won't untap during your next untap step.)

Resolute Survivors

You may exert Resolute Survivors as it attacks. (It won't untap during your next untap step.)
Whenever you exert a creature, Resolute Survivors deals 1 damage to each opponent and you gain 1 life.

Smoke

Players can't untap more than one creature during their untap steps.

Hydra Trainer

You may exert Hydra Trainer as it attacks. When you do, target creature gets +X/+X until end of turn, where X is the number of counters on permanents you control. (An exerted creature won't untap during your next untap step.)
{2}{G}: Adapt 2. (If this creature has no +1/+1 counters on it, put two +1/+1 counters on it.)

Hooded Brawler

You may exert Hooded Brawler as it attacks. When you do, it gets +2/+2 until end of turn. (An exerted creature won't untap during your next untap step.)

Glorybringer

Flying, haste
You may exert Glorybringer as it attacks. When you do, it deals 4 damage to target non-Dragon creature an opponent controls. (An exerted creature won't untap during your next untap step.)

Sandstorm Crasher

Trample
You may exert Sandstorm Crasher as it attacks. When you do, create a tapped and attacking token that's a copy of target creature you control. Sacrifice the token at the beginning of the next end step. (An exerted creature won't untap during your next untap step.)

Vizier of the True

You may exert Vizier of the True as it attacks. (It won't untap during your next untap step.)
Whenever you exert a creature, tap target creature an opponent controls.

Tah-Crop Elite

Flying
You may exert Tah-Crop Elite as it attacks. When you do, creatures you control get +1/+1 until end of turn. (An exerted creature won't untap during your next untap step.)

Combat Celebrant

If Combat Celebrant hasn't been exerted this turn, you may exert it as it attacks. When you do, untap all other creatures you control and after this phase, there is an additional combat phase. (An exerted creature won't untap during your next untap step.)

Oketra's Avenger

You may exert Oketra's Avenger as it attacks. When you do, prevent all combat damage that would be dealt to it this turn. (An exerted creature won't untap during your next untap step.)

Anep, Vizier of Hazoret

Trample
You may exert Anep, Vizier of Hazoret as it attacks. When you do, exile the top two cards of your library. Until the end of your next turn, you may play those cards. (An exerted creature won't untap during your next untap step.)

Battlefield Scavenger

You may exert Battlefield Scavenger as it attacks. (It won't untap during your next untap step.)
Whenever you exert a creature, you may discard a card. If you do, draw a card.