An idiom meaning to be on very bad terms with someone — a relationship of hostility (literally 'dogs and monkeys').
あの二人は学生時代から犬猿の仲だ。
Those two have been on very bad terms since their student days.
会社では、部署長と係長が犬猿の仲で雰囲気が悪い。
At the company, the department head and the subsection chief are on very bad terms, and the atmosphere is bad.
昔から犬猿の仲と言われている兄妹だが、時々助け合っている。
Although the siblings have long been said to be on very bad terms, they sometimes help each other.