It is a strange sentence, and a poor translation. The literal translation is "he fights against animals", so you are correct -- it is unclear why the preposition "against" is not in the above english answer. I would assume it's an error. Duo did accept "he fights against animals" as a correct response.
A quick google shows a Slate Afrique headline which states "Un Egyptien combat un lion", so it seems that "il combat des animaux/he fights animals" would be grammatically correct in French without the preposition.