Feral Contest

If just the first targeted creature is an illegal target by the time Feral Contest resolves, it won’t get a +1/+1 counter. However, the second targeted creature is still affected by the blocking restriction the spell imposes, so it’ll have to block the first targeted creature that turn if able.

If just the second targeted creature is an illegal target by the time Feral Contest resolves, the first targeted creature will get a +1/+1 counter, but the second targeted creature won’t have to block it that turn.

You must choose two targets as you cast Feral Contest: a creature you control and any other creature. If you can’t (because there’s only one creature on the battlefield, perhaps), then you can’t cast the spell. Note that the second target may also be a creature you control.