In the old way of doing things, we treat our servers like pets, for example Bob the mail server. If Bob goes down, it’s all hands on deck. The CEO can’t get his email and it’s the end of the world. In the new way, servers are numbered, like cattle in a herd. For example, www001 to www100. When one server goes down, it’s taken out back, shot, and replaced on the line.
"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.