A batman or an orderly is a soldier or airman assigned to a commissioned officer as a personal servant. Before the advent of motorized transport, an officer’s batman was also in charge of the officer’s “bat-horse” that carried the officer’s kit during a campaign. The British English term is derived from the obsolete bat, meaning “pack saddle” The military term long predates the appearance of the fictional superhero Batman.

https://en.wikipedia.org/wiki/Batman_(military)