Kraul Harpooner

Reach
Undergrowth — When this creature enters, choose up to one target creature you don't control with flying. This creature gets +X/+0 until end of turn, where X is the number of creature cards in your graveyard, then you may have this creature fight that creature.

Pioneer Masters

The List

Ravnica: Clue Edition

Reach
Undergrowth — When Kraul Harpooner enters the battlefield, choose up to one target creature you don't control with flying. Kraul Harpooner gets +X/+0 until end of turn, where X is the number of creature cards in your graveyard, then you may have Kraul Harpooner fight that creature.

Jumpstart 2022

Reach
Undergrowth — When Kraul Harpooner enters the battlefield, choose up to one target creature with flying you don't control. Kraul Harpooner gets +X/+0 until end of turn, where X is the number of creature cards in your graveyard, then you may have Kraul Harpooner fight that creature.

Guilds of Ravnica

Because tokens aren't cards, they never count for undergrowth abilities.

Creature cards with other types, such as artifact creature cards, count for undergrowth abilities.

If you choose a target and the target creature is an illegal target when Kraul Harpooner's ability tries to resolve, the ability doesn't resolve and Kraul Harpooner doesn't get +X/+0. If the target creature is legal but Kraul Harpooner is no longer on the battlefield, the target creature won't deal or be dealt damage.

If you don't choose a target creature, Kraul Harpooner simply gets +X/+0 until end of turn.

The value of X is determined only as the undergrowth ability resolves. If the number of creature cards in your graveyard changes later in the turn, Kraul Harpooner is unaffected.

You choose the target of the triggered ability (or that it has no target) as it goes on the stack, but you choose whether the creatures fight as that ability resolves.