Affectionate Indrik

When this creature enters, you may have it fight target creature you don't control. (Each deals damage equal to its power to the other.)

The problem isn't the indrik's enthusiasm, it's Ravnica's lack of guardrails.

Foundations

Ravnica: Clue Edition

When Affectionate Indrik enters the battlefield, you may have it fight target creature you don't control. (Each deals damage equal to its power to the other.)

Arena Beginner Set

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

Jumpstart

If the target creature is an illegal target when Affectionate Indrik's ability tries to resolve, the ability doesn't resolve. If Affectionate Indrik is no longer on the battlefield, the target 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 the creatures fight as that ability resolves.