User blog comment:Mmpratt99 deviantart/If there was an actual biologically plausible monster that hunted humans, what would it look like?/@comment-17660443-20180528213523

Hmm... well historically, our ancestors (and modern apes) were hunted by big cats and sometimes large birds of prey. There are some more recent historical records of big cats like tigers going after humans over other food sources too. For the most part though, primates are killed by other primates, either within the same species or by a larger/stronger or more intelligent one.

Given the kind of firepower humans tend to bring out when threatened, a primate-like predator seems most likely; any of the above three categories could probably be moderately successful though, given the right habitat and lifestyle. Living somewhere that people fighting back wouldn't be able to reach easily, and probably having a solitary social structure rather than a communal one, to minimize the chances of them being wiped out in one fell swoop, could help.

So tl;dr: a big cat, large enough bird of prey, or strong/smart enough primate that lives somewhere remote or secure and is probably solitary. Or in other words, a lot of the popular cryptids of folklore!