Special forces as we think of them did not exist in ancient or medieval armies. While there were units that were considered particularly elite (due to prestige, equipment, experience, or whatever), these were not equivalent to modern special forces. Modern special forces are often defined by their role: they carry out unconventional warfare on behalf of a conventional army. In premodern warfare, the distinction between conventional and unconventional warfare did not exist. Historians today will sometimes refer to particular campaigns or commanders as examples of pre-modern "guerilla warfare," but this is an anachronistic and misleading way of viewing combat. Undoubtedly someone will come along in this thread and quote examples of Alexander the Great using a small group of men to scale a wall or some similar anecdote. You'll notice, however, in all of these anecdotes, the group described is not a permanent entity. There is no such thing as Alexander's 34th Wall Climbing Brigade. Groups like that, created in response to an immediate need, were temporary and dissolved after their purpose was fulfilled.