Ancient Animus

• WOTC Official Reminder Textofficial rules
  Fight — Each of those creatures deals damage equal to its power to the other.

• WWizards of the Coast2020-11-10
  Official Ruling
  If the creature you control is an illegal target as Ancient Animus tries to resolve, you won't put a +1/+1 counter on it. If that creature is a legal target but the creature you don't control isn't, you'll still put the counter on the creature you control if it's legendary.
• WWizards of the Coast2020-11-10
  Official Ruling
  If either target is an illegal target as Ancient Animus tries to resolve, neither creature will deal or be dealt damage.
• WWizards of the Coast2020-11-10
  Official Ruling
  You can't cast Ancient Animus unless you choose both a creature you control and a creature you don't control as targets.
• WWizards of the Coast2020-11-10
  Official Ruling
  The creature you control doesn't have to be legendary. It simply won't receive a +1/+1 counter before it fights if it isn't legendary.

Rulings via Scryfall