YA Fantasy. MMC enters into a competition where each individual has an animal sidekick. The MMC has a fox, I think it was in a jungle. The big "twist" was that one character had a small boy as their sidekick instead of an animal.

[Gretchen Burnette]

MMC enters into a competition where each individual has a different animal sidekick with special powers. The MMC went into the competition willingly to help his family, and he is given a fox (with lightning?), I think it was in set a jungle. The big "twist" was that one character had a small boy as their sidekick instead of an animal, and the little boy jumped into a raging river to save his person. Everyone has to get to this end space and the first one wins, but a group of at least three forms to travel together. I read the book around 2014, and the last of the series hadn't yet been published, so 2000-2015 I'd say. I know the covers of the books were mostly black, one with some yellow accents and the other with blue accents swirling, maybe a face. The title had at least three words. Very obscure series, I've never heard of anyone outside of my teen book club who knew it and the series went unfinished I believe.