Humans versus animals? It could very well happen in 50 or 100 or 500 years as society continues to evolve.
So, what if a fighter trained in MMA fought against the King of the Jungle, a/k/a the Siberian Tiger?
The tiger would obviously have to be de-clawed so it couldn't stick its nails into the fighter's ribs and guts. I'd say that it would also make sense to sand down the tiger's teeth so the tiger would not be able to crunch into the fighter's bone, but still sharp enough to grind the bone (if you've ever been bit by a tiger, then you understand the difference).
Who would win?
I guess I should add that I'm not talking about a death match. The fight would be three, five-minute rounds, just like UFC.