The standard answer is "no, there were no special forces units in the middle ages," but the way you phrased this requires additional clarification. There were small units (sometimes of just a few dozen men, sometimes several hundred or even a thousand) who would split off from the main army to raid and scout. Small unit actions were also very common in the kind of low-intensity but near-constant warfare that occurred in frontier regions or particularly contested locales. But these weren't considered "special operations" by medieval commanders. They were the same sorts of things that the main army was also engaged in. There would not be special detachments for raiding enemy villages or for capturing prisoners for interrogation. The bulk of such activity was often carried out by mounted troops, but again, there was no specialized force trained and dedicated to doing so. Sometimes, a group of experienced and skilled soldiers might be assembled in order to carry out a specific task, but these were pretty much always organized on the spur of the moment. At Agincourt, two wings of experienced French knights were picked to sweep the flanks clear of English archers, but the terrain and English field entrenchments saw them off relatively easily. In the same battle, another group of French aristocrats voluntarily came together and swore to kill or capture King Henry V. This resulted in the deaths of all 18 men involved.