Brock Lesnar is a large man but he couldnt even beat The Rock on his own. He needed help from Heyman and even a chair.
Anderson Silva is a great fighter in his own right but he isnt as large as Brock Lesnar and that alows for quicker movements.
Putting Silvas fighting style with Lesnars size would actually hinder his ability to fight as well as Silvas normally.
A Siberian Tiger can weigh up to 700 pounds and can run up to 60mph. With natural ferocity, weapons, and hunting ability it is incredibly hard to see any unarmed man take one down with his bare hands. Not to mention it is considered the Strongest land animal on earth.
A Polar Bear can weigh up to 1500 pounds and reach speeds of 25 mph. Though slower then the Tiger they are still incredibly strong and can kill an animal of 1200 pounds with a single hit. This is another animal I dont see any human fighting unarmed and winning.
A Grizzley Bear is closley related to a Polar Bear but the Polar Bear is actually considered stronger. The Fact of the Polar Bear able to kill an animal of around 1200 pounds with a single hit came from the Grizzly. Another Animal that can not be beaten by an unarmed man.
Silver Back Gorillas weigh up to 400 pounds and have the highest Mix of Strength and Dexterity over any animal. They have been known to be able to bend tempered steel bars and snap thick stalks of bamboo with little effort. A man has a better chance to beat the Tiger then he does beating a Silverback Gorilla.
The Wolf is the only animal in the list that a human might have a chance against in an unarmed fight. A Wolf is a pack hunter and uses Distraction tactis in order to take down its prey. A single wolf may be easily taken down by someone who is quick and strong.
Siberian Tiger Vs Man? No
Polar Bear Vs Man? No
Grizzly Bear Vs Man? No
Gorilla Vs Man? No
Wolf Vs Man? Maybe.