牡 is a bound form (不成词语素) that cannot stand alone. It appears in compound words with two distinct functions: it means 'male' in animal terms like 牡牛 (bull) and 牡马 (stallion), and it also serves as a phonetic component in loanwords such as 牡蛎 (oyster) and 牡丹 (peony), where it carries no independent meaning. You'll only encounter it as part of a larger word.