Sure. In the Byzantine Empire alone, there were a number of units that could be considered "special forces". And by "special forces", I'm assuming you mean an elite unit that partakes in things like "covert ops" concerning matters of extreme importance to the security and well-being of a state.

The Scholai (from Scholae Palatinae, meaning "Palatine Schools") and the Excubitors (from εξκουβιτοι, meaning "sentinels"), were two elite Romano-Byzantine units of Late Antiquity and the Early Medieval Period, that served a variety of military roles throughout their respective histories, including: elite guards, saboteurs, spies, enforcers of internal security, ambassadorial escorts, shock troops, etc.

Furthermore, although perhaps not explicitly stated which units were used to partake in the operation, Anna Komnene's Alexiad elaborates on a series of amphibious assault raids by Byzantine forces (under orders from Emperor Alexios I Komnenos), designed to disrupt Turkish operations on the Anatoline coast. You can read a brief elaboration of these sorties here in Section X of the following book chapter. I find it very easy to draw parallels between this description and descriptions of modern amphibious special forces like the SEALS, SBS, etc.

I'm not confident at all talking about the whole of Western medieval warfare, but as no-one else has yet, I'll jump in until a more knowledgeable subscriber gets involved.

Bearing in mind that the concept of a professional army only took a decent hold in Western thought after the invention and fairly widespread takeup of the gun, the short answer is no, this would be very unusual. Your best-trained and equipped men would be nobleman, and for reasons of chivalry and practicality they usually be involved in mounted cavalries which would fight en masse.

Taking a broad definition of what you're after, I can think of a potential unit which fit some characteristics of being;

• Well trained

• Mobile

• Given specific roles not involving the main grind of battle

• Full-time soldiers

The Hungarian Black Army was one of the first full-time standing armies of Europe. Their hussars were mounted light cavalry units that would scout, disrupt and skirmish ahead of the main army; this wasn't a tactic unique to the Black Army but this is the only unit I can think of in a standing army . p81 p246&247

I was tempted to mention the English Order of the Garter, as it was a collection of elite British knights during the Hundred Years War selected as much for fighting ability as nobility, but I don't think they really translate, as the members would often be used as commanders or integrated into the main army forces.

The problem is, modern special forces when deployed in war are usually elite soldiers doing secretive commando-style operations. In the medieval time period, your elite soldiers - usually nobles in heavy armour on horseback - would be expected to be part of the main teeth of your army.

The only other potential answer to this question I could think of would be an occasion when a siege was resolved by the actions of a small group of infiltrators. I've racked my brains and I think there's at least one example of this but can't put my finger on it.

I watched the movie Lone Survivor last night and that's what this post / thought came from.

You may want to check out this post: Did ancient military's have "elite" units like the SAS or SEAL Team Six of today?