The classic question.
Rules: This is a fight in an arena, you are unarmed, you cannot leave the arena until every other animal is dead, and while the animals are as intelligent as normal they do understand that they’re fighting you and will try to do so, they won’t just run away.
I wouldn't be able to kill a horse-sized anything with my bare hands so it must be the 100 duck-sized horses.
Also, a horse-sized duck would be a nightmarish abomination whereas duck-sized horses would actually be quite cute.