You might be interested in some previous threads about special operations forces and elite units in pre-modern militaries:

Are pre-modern special forces anachronistic?

• During your time period, what were nation's "Special Forces" or clandestine operations like and what were their objectives? - 27 comments, over 2 years old.

• This thread gets into the nature of modern special forces and details how pre-modern elite units had very different goals and may not be appropriately called special operations forces.

• Did dark/middle age armies have any type of special forces units? - 20 comments, over 1 year old.

• The topmost comment summarizes why it might be inappropriate to look for examples of special operations forces in medieval militaries.

• Did Ancient Rome or Medieval Europe have some form of "Special Forces"? - 16 comments, over 7 months old.

• The flaired commenters here list criticisms of attempts (including a published book) to shoehorn modern concepts of special operations forces onto pre-modern military units.

• When were "Special Forces" first used? - 7 comments, over 2 years old.

• This delves into the modern genesis of special operations forces with some ancient examples used to demonstrate they're differences in comparison with modern units.

Elite Roman Units

• Did the Roman Army have "Special" Units? - 9 comments, over 8 months old.

• A commenter here provides a detailed run-down of specialized Imperial Roman forces and their roles from the during the reign of Augustus and into the fourth century.

• Did the Romans have 'Elite' units, similar to our special operations forces? - 43 comments, over 4 months old.

• Examples are brought up amongst the Roman legions and auxillaries with a debate in the top comment thread about whether any of them qualify as special operations forces

• Did the Roman army ever have any "special forces" type units? - 37 comments, over 10 months old.

• Various legions, auxillaries, and cavalry forces are brought up.

• Were there any "elite" Roman Auxiliaries? - 14 comments, over 2 years old.

• A variety of auxillary units are briefly described here.

Elite Medieval Units

• Were knights on foot feared by normal men-at-arms? Were they in a sense "elite" fighters and avoided in combat? - 157 comments, over 9 months old.

• This thread is filled from top to bottom with in-depth answers about various elite warriors and groups in the Middle Ages as well as how they compared with more common warriors available.

• Were the Templar Knights particularly tough bad-asses? - 139 comments, over 1 year old.

• This gets into the training, battlefield performance, duties, and equipment that these knights used and whether or it qualifies them as special operations forces in the modern sense.

• Was there ever a medieval Special Forces/Navy Seal type small team units? - 10 comments, over 1 year old.

• The topmost comment discusses Byzantine units and services that fall under this category while a top level comment below it goes into detail on a Medieval Hungarian example that may qualify.

• Were there any examples of 'special forces' in Medieval and Ancient times? - 6 comments, over 1 year old.

• The sole top-level comment puts various types of Late Medieval Western European soldiers in perspective to show why they don't particularly qualify as special operations forces.

Elite Early Modern Units

• During the time of the British empire, was there any type of elite group of soldiers akin to modern day navy seals? - 7 comments, over 1 month old.

• A flaired commenter discusses the roles of various British infantry units during the American Revolutionary War and the many conflicts with Napoleon.

• In the Napoleonic Wars, what was the difference between a soldier in an elite unit and a regular unit? - 5 comments, over 5 months old.

• The commenters in this thread give detailed rundowns of the qualifications, training, and roles of the French Imperial Guard with particular emphasis on the Old Guard.

• Was there an equivalent of the modern military "Special Forces" in the past? - 9 comments, over 1 year old.

• Below the topmost comment which discusses the Sacred Band of Thebes, a commenter compares the role and use of light cavalry during the Napoleonic Wars to modern day special operations forces.

Other Examples

• Did ancient military's have "elite" units like the SAS or SEAL Team Six of today? - 88 comments, over 1 year old.

• A variety of examples are brought up from Antiquity as well as from Pre-Columbian Mesoamerica and Warring States China.

• Was their any specialized forces in ancient armies such as the seals or CIA? - 5 comments, over 8 months old.

• A commenter speaks about the roles and famous acts of ninja during the Sengoku Period