Are there any records or myths of a Medieval or Ancient equivalent to modern special forces? Defined as small groups operating outside of the main army, specialized in high risk operations such as infiltrating enemy camp, neutralizing high value targets, such as commanders or siege engines, supplies or horses?
Did armies in that time period ever regularly conduct precision skirmishes, aiming at such important targets? Were there any units especially trained for that purpose in war?
In terms of medieval armies, I can only speak about the English, French and Scottish armies who participated in the wars between 1200-1400. My primary source for this knowledge is, once again, John Keegan's Face of Battle and Jonathan Sumption's The Hundred Years War: Trial by Battle.
But no. At best, you might ask a portion of a Lordly retinue or Knights Bachelor to go out and assail a town, or detail a set number of troops to raid and attack the surrounding territory (which led to bands of longbowmen being known as 'Hellequin' by the French). The reason for this is that they did not utilise standing armies; while there would be well-trained soldiers, the vast majority of these would have duties and professions outside of war. Archers might be butchers, huntsmen, fletchers, messengers or farmers during peacetime, and Men-at-Arms often were administrators and stewards for nobility. While the Late Medieval Period is sometimes seen as an incredibly violent one, for the average soldier, there was no need to acquire the skills that might facilitate 'special forces' tactics. On the battlefield, large banners might be specifically targeted. This is not a source, an old story about Agincourt illuminated by Juliet Barker is that 20 French Knights swore to kill or capture Henry V before the battle, and all were killed attempting to do so.
Also, it must be pointed out that attacking the supplies of a medieval army on campaign would not be terribly effective; the wagons would be spread amongst the army, and most relied on foraging for large portions of their sustenance, as Livingstone and Wittzel point out in The Road to Crecy. However, sallies were attempted against siege engines from the walls, and this occured at the Siege of Harfleur in 1415.
The closest you could ever really get MIGHT be the groups of 'Ulfsarks' described in the saga of Egil Skallagrimson, who would join together before going into a horrendous fury and then throwing themselves at the enemy.
My general conclusion from my reading, and supported by my professors, is that medieval armies had neither a driving need nor the military infrastructure to produce highly trained individuals capable of performing the tasks you laid out.