Despite the reemergence of werewolves and the proliferation of Eldrazi, elk numbers in the Somberwald have remained steady.

Somberwald Stag

When this creature enters, you may have it fight target creature you don't control.

Jumpstart

Eldritch Moon

When Somberwald Stag enters the battlefield, you may have it fight target creature you don't control.

If the targeted creature is an illegal target when Somberwald Stag’s ability tries to resolve, the ability doesn’t resolve. If Somberwald Stag is no longer on the battlefield, the targeted creature won’t deal or be dealt damage.

You choose the target of the triggered ability as it goes on the stack, but you choose whether or not to fight as that ability resolves.