"Cattle" can only be used in the plural and not in the singular: it is a plurale tantum. Thus one may refer to "three cattle" or "some cattle", but not "one cattle". "One head of cattle" is a valid though periphrastic way to refer to one animal of indeterminate or unknown age and sex; otherwise no universally used single-word singular form of cattle exists in modern English, other than the sex- and age-specific terms such as cow, bull, steer and heifer.