In one corner you have a human adult male - fit, trained fighter, no weapons.
Now you want to pit him against some kind of animal in a cage fight to the death. But you don’t want a big predator that will tear him apart easily (like a big cat or a chimp), nor do you want an animal that’s too weak to pose a serious threat. We need to give the audience some entertainment for their money. So what animal do we pit against him that would be more or less equally matched?