msem-instigator

Whenever Wild Dogs of the Night or another Horror you control enters, exile up to one target card in a graveyard. If that card was a creature card, you gain 2 life and put two +1/+1 counters on that Horror.

Frenzy 1 (Whenever this creature attacks and isn't blocked, it gets +1/+0 until end of turn.)
Wild Haploc has first strike as long as it's attacking.

Colossal 4—{5}{G} (You may cast this for its colossal cost to have it enter with four +1/+1 counters; or pay {5}{G} as a sorcery: Put four +1/+1 counters on it if it has none.)
Trample

Improve {G} ({G}: Put a +1/+1 counter on this creature. This costs {1} more to activate for each +1/+1 counter on it. Improve only as a sorcery.)
If a +1/+1 counter would be put on a creature you control, you may put a deathtouch, reach, trample, or vigilance counter on it instead.

Assault {2}{G} (Whenever this attacks, if it has no +1/+1 counters on it, you may pay its assault cost to put a +1/+1 counter on it.)
When Wildborn Marauder assaults, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Harmony — You may pay {1}{G}. If you do, create a 3/3 green Beast creature token.
When Wildheart Chorister enters, harmonize. (Trigger up to one harmony of a permanent you control.)

Trample
As long as you control an Elemental creature, Wildheart Gorran gets +1/+1.

{1}{G}, Remove a -1/-1 counter from a creature you control: Wiltthorn Elemental gets +2/+2 until end of turn.

Flash (You may cast this spell any time you could cast an instant.)
When Wind-Blessed Guardian enters, you may return target permanent you control to its owner's hand.

Flying
Revive {1}{B}{B} ({1}{B}{B}, Exile this card from your graveyard: Create a token that's a copy of it except it has no mana cost. Revive only as a sorcery.)

Flash
When Wiretapper enters, choose one —
• Exile target spell. For as long as that card remains exiled, its owner may play it. A spell cast this way costs {2} more to cast.
• Create a copy of the card with target spell's name. You may cast the copy. (You still pay its costs.)

Flying

When Wistful Fox enters, reveal the top eight cards of your library. You may put a creature card from among them into your hand. Put the rest on the bottom of your library in a random order.

Deathtouch
{T}: Add one mana of any color.

Menace
Crew 2 (Tap any number of creatures you control with total power 2 or greater: This Vehicle becomes an artifact creature until end of turn.)

Whenever Withering Husk enters or attacks, you may discard a card and pay 1 life. If you do, draw a card, then each opponent loses 1 life.
Recite {R} (You may cast this creature from your graveyard for its recite cost. If you do, exile it when it enters or if it would leave the battlefield.)

Destroy all non-Horror creatures.
Coalesce {8}{W}{W} (You may cast this from your graveyard for {8}{W}{W}. If you do, it enters as a Horror creature after it resolves. Exile it if it would leave the battlefield or not resolve.)

When Wolfbriar Sapling enters, if you control seven or more lands, create a 2/2 green Wolf creature token.

When Wondering Oaf dies, return target instant or sorcery card from your graveyard to your hand.

When Woopip Bruiser enters, germinate. When you do, you may have target non-Fungus creature get -X/-X until end of turn, where X is the greatest power among creatures you control. (To germinate, put a +1/+1 counter on a creature or create a 1/1 green Saproling creature token.)

When Woopip Sproutling enters, germinate. (Put a +1/+1 counter on a creature or create a 1/1 green Saproling creature token.)

Whenever an artifact you control enters or is put into a graveyard from the battlefield, Workshop Engineer gets +1/+1 until end of turn.

{T}, Sacrifice Worldshaper Adept: Choose two basic land types. Create a tapped colorless land token of the chosen types.

Flash
When Wormwood's Agent dies, create a Vial token. (It's an artifact with “{1}, Sacrifice this and another permanent: Draw a card. Activate only during your turn.”)