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.
Duck sized horses for sure. The weaknesses of a duck (thin neck and legs) are shored up by being made larger while the strengths of a horse (the powerful bite and kicks) are nearly fully negated by being shrunk down. I'd rather fight a horse sized duck than a horse sized horse though.