cow's rump
Literal meaning, rarely used outside the idiom 鶏口牛後 (better to be the head of a chicken than the rear end of a cow).
low-ranking member of a large group; small fish in a big pond
Figurative use, always in the context of the proverb 鶏口となるも牛後となるなかれ (rather be the head of a chicken than the tail of a cow). Implies being an insignificant follower in a large organization.
See also: 鶏口 · 鶏口牛後 (けいこうぎゅうご)
牛後 literally means a cow's rump, but it is almost never used by itself.
Rather than being a small fish in a big company, I'd rather aim to be a big fish in a small one.