牛(niú) - cow, ox, cattle in Mandarin

牛 (niú) is the general word for bovine animals. Chinese does not split cow, bull and ox the way English does, so 牛 covers all of them and you add a word when the distinction matters: 母牛 for a cow, 公牛 for a bull, 奶牛 for a dairy cow. The measure word is 头 (tóu), literally a head, exactly as English farmers count head of cattle.

It builds a lot of everyday vocabulary. 牛肉 is beef, 牛奶 is milk, and 牛仔裤 is a pair of jeans, borrowed via the cowboy.

The surprise for a learner is that 牛 is also modern slang for impressive or formidable. 他很牛 means he is seriously good at something, and the stronger 牛的 versions are vulgar, so keep to the plain adjective. The praise is genuine, not sarcastic, which catches out English speakers who expect anything comparing a person to livestock to be an insult.