Biggest animal you'd fight for money

[bobman0330]

Say a billionaire weirdo wants to see people fight animals for fun. He's offering you $50 million to enter a closed arena with a starving, mean animal of some sort. Three scenarios: a) you're unarmed; b) you have a baseball bat; c) you have a revolver with 3 bullets. What's the scariest animal you'd be willing to fight?

I debated making a poll, but it's too much work, and there might be some debate about the hierarchy. Here are some animals to get you thinking:
Cat
Dog
Horse (assume he wants to attack you for some reason)
Wolf
Lion
Reasonable-size bear
Tiger
Elephant