In the somewhat far future, mankind has become fairly adept at wet nanotechnology and synthetic biology. Allowing them to not just modify what exists in nature, but build beyond it with almost anything organic.
When I say organic, I mean organic, there are more than a few organisms that incorporate what we'd recognize as plastics and synthetic fibers into their biology. They're also far better at precipitating metal than real life organisms. Which is kind of why I felt this belonged with a xenobiology tag. So naturally gladiatorial arenas are in vogue to pit fantastic beasts against each other.
No expense is spared in creating these masterworks of bioengineering. Bio-electronics, optics, synthetic muscles far stronger than anything in nature could evolve. Their size varies as well from being between the size of a wolf to being no bigger than a Siberian Tiger.
However one major problem is that these organisms need a lot of calories to fight and perform for their owners. But I've also read once that a gallon of gasoline contains 31,000 calories.
Is there any plausible mechanism to get these organisms to digest and breakdown fossil fuel products such as gasoline or kerosene for energy?