猿猴 is a category term - "apes and monkeys / primates," not a single species. Strictly:
猿 (yuán) = tail-less great apes (gorillas, gibbons);
猴 (hóu) = the tailed monkeys you see in Chinese zoos. Both characters share the
犭 (dog/beast) radical, the standard signal for an animal-class character.
