牛 is an ox seen head-on: the vertical is the face, the horns sweep up from the top stroke. One of the oldest and least-changed characters — an ox has looked like 牛 for 3,300 years.
牛 plowed Chinese civilization and now compliments it: 牛肉 (beef), 牛奶 (milk), and the slang triumph 你真牛! ('you're awesome!' — you are truly OX). The zodiac ox is diligence itself; 老黄牛 praises tireless workers; a bull market is a 牛市. Why is 'ox' awesome? The full slang is earthier (牛屄), but polite company just says 牛 — and everyone grins.
Horns up, face forward — the OX (and anyone as strong as one).
牛市/熊市 (bull/bear market) matches English exactly — global finance speaks zodiac.