msem-instigator

When Horn of Seasons enters, search your library for a basic land card, put it onto the battlefield tapped, then shuffle.
Equipped creature gets +3/+0.
Equip {3} ({3}: Attach to target creature you control. Equip only as a sorcery.)

Flying
When Horned Spy enters, look at target player's hand.

No more than two creatures can attack each combat.

Trample
Whenever Horrific Progenitor deals damage to a player, for each damage dealt, that player loses 3 life unless they sacrifice a nonland permanent or discard a card.

Haste
When Horse Thief enters, you may gain control of target creature or Steed until end of turn. It gains haste until end of turn. If it's a Steed, attach it to Horse Thief.

Flying, lifelink

When Host of Nature enters, create two 2/2 red and green Elk creature tokens.

Whenever you cast a noncreature spell, any number of target creatures you control get +1/+1 until end of turn.
Whenever a creature you control deals an amount of combat damage exactly equal to its mana value to a player or planeswalker, draw a card.

Tap an untapped Desert you control: Hostile Atmosphere deals 1 damage to any target.

Flying
When Hostile Barbfly enters, you may have it consume. When you do, target creature gets -2/-2 until end of turn. (To consume, sacrifice another creature or exile a creature card from your graveyard.)

Alpha (You may have this creature enter with a +1/+1 counter on it. If you do, it becomes alpha and can only attack alone.)
Lifelink

Flash
When Hostile Takeover enters, you may counter target noncreature spell.
At the beginning of your upkeep, you may put a progress counter on a Project you control.

{T}: Hotblooded Sorcerer deals 1 damage to target player or planeswalker.
Attain {3}{R} ({3}{R}: Transform this. Spend at least three colors of mana to activate this, and only as a sorcery.)

Whenever you make first contact, Hotshot Renegade deals 2 damage to up to one target creature or planeswalker. (You make first contact the first time each turn you cast a spell with a name not in your graveyard or control already.)

Renown 1 (When this creature deals combat damage to a player, if it isn't renowned, put a +1/+1 counter on it and it becomes renowned.)
Whenever a creature you control becomes renowned, scry 1.

{T}: Target player exiles a card from their graveyard. If they do, you gain 1 life.

{T}: Add {C}.
{1}, {T}, Sacrifice House of Forgotten Whispers: Draw a card.

Flying
When Hoverbike Courier enters, you may reappropriate. If you do, draw a card. (To reappropriate, exile exactly two cards from your hand and/or graveyard.)

Advance {2}{W} (Cast this for {2}{W} to advance it as it enters. {2}{W}: Advance this as a sorcery.)
Whenever Hoverstepper advances, target creature gets +1/+1 and gains flying until end of turn.

When Howler Sellout enters, for each creature that died this turn, post a bounty on up to one target creature an opponent controls. (Put a bounty counter on that creature. It has “When this creature dies, each opponent draws a card” as long as it has a bounty counter on it.)

When Howler Sheriff enters, gain control of up to one target creature with a bounty counter on it until end of turn. Untap that creature. It gains haste until end of turn.

Menace
Whenever an opponent activates an ability of a permanent, if it isn't a mana ability, Howling Shieldbreaker deals 2 damage to that permanent or player.

Hozen Shrine enters tapped.
{T}: Add {U} or {B}.
{U/B}, {T}, Sacrifice Hozen Shrine: Put a +1/+1 counter on target creature you control. Activate only as a sorcery.

Advance {3}{W} (Cast this for {3}{W} to advance it as it enters. {3}{W}: Advance this as a sorcery.)
Whenever Hrexa Enforcer advances, put a +1/+1 counter on it.

Transcend (Activate as a sorcery to gain the next type and a +1/+1 counter.)
{T}, Tap an untapped creature you control: Add one mana of any color.
{G}: Demigod — Whenever you cast a creature spell this turn, draw a card.
{4}{G}{G}: God — Creatures you control get +3/+3 and gain trample until end of turn.