牡

Left ox radical - compressed left-side form of 牛, with the bottom horizontal stroke angled up-and-out for vertical fit. Indexes 牡 in the cattle family: 牧 (herd), 物 (livestock variety), 特 (single male ox), 牲 (sacrificial animal). 牡 originally meant specifically a male ox; from that bovine origin it generalised to mean male of any animal: 牡蛎 (male oyster), 牡马 (stallion).

Right 土 - here historically not the "earth" radical but a phallic marker, originally a pictograph of male genitalia (later reanalysed as "earth" in other contexts). Combined with the ox on the left, the character gave a literal compound ideograph: a male bovine. The female counterpart 牝 uses 匕. Together 牡牝 = male and female animals.