Back in the times of the Great Crusade, the Legion's recruitment differed from what current day Chapters do. Once a recruit passed their training, they got their armor and gear (just like Grey Knights and Space Wolves practice), thus there were no Scouts. Recon Squads are roided-up Scouts with power armour, sniper rifles, cameleoline and a set of gear like the Nuncio Vox (though they can take other weapons like bolters, shotguns and melee weapons). Military experts trained in the art of sabotage, raiding, intel-gathering, assassination, stealth and recon deep behind enemy lines, these guys usually don't go head-on in a fight. Yet when there is no room for stealth and there was a open battle, these individuals quickly switched to flanking maneuvers and infiltration attacks while supporting their main forces.
After the Horus Heresy and the splitting of the Legions into smaller Chapters during the 2nd Founding, the modern-day Scouts emerged replacing the Reconnaissance Squads completely. In the 42nd Millenium after the introduction of the Primaris Marines, this unit actually got an actual spiritual successor in the form of Eliminator Squads of the Vanguard Primaris.
A Raven Guard-specific formation during the Great Crusade and Horus Heresy, the Mor Deythan (aka: Shadow Masters) are essentially to Reconnaissance Squads the same way the latter are to Scouts. The origin of these badasses stretches back to the times when Corax landed on Lycaeus (how Deliverance used to be called) and when he formed the repressed population there into his army of freedom fighters to battle the brutal overlords of Kiavahr. After winning independence for the now renamed Deliverance, and the collapse of Kiavahr due to civil war that resulted between the Tech-Guilds, the Emperor arrived and revealed to Corax his origins. The Primarch of the XIXth Legion, now renamed the Raven Guard, took with him into his new army a number of rebels who wanted to stay with him. From among them, the strongest of youths were turned into Legionnaires. It were those Marines from which the Mor Deythan would emerge.
The Astartes that would make up the Mor Deythan were known to be effective assassins and masters of infiltration (hence their name Shadow Masters), as well as so secret that even the other members of the Raven Guard didn't know they even existed. They could do the same tasks as Reconnaissance Squads did, yet with skills and capabilities surpassing those of the mainline Recon Squads. Each one of these warriors were veterans of the Lycaeum Uprising who fought with Corvus side-by-side (hence why they rarely took in new recruits). Apart of that and their incredible skills, they share one unique trait with their Primarch whom only the latter and the Apothecaries know about. The Mor Deythan managed to inherit with the geneseed of Corax the unique ability the Primarch only had, namely the Shadow-Walk ability that resulted in them erasing their presence from the minds of the enemy warriors the same way Corax does. This, along with their skills, gear and experience, made this elite formation a true nightmare for anyone who faced the Raven Guard. They even had their own battle-cant called the Stalk Argot which considered of a combination of signs and whispers that were nigh-indistinguishable from wind and environmental sounds.
During the Horus Heresy however, the Mor Deythan suffered large casualties during the Dropsite Massacre on Istvaan V, yet despite this they fought on numerous battlefields. Attrition started wear them out to the point a few dozen remained (due to them being spread thin through the galaxy at the time when that happened), but those that still were on active duty were even more grizzled than usually and started to exhibit abilities they previously did not. Some even said that somehow they inherited a portion of their Primarch's genetic capabilities. As befitted of an elite formation like the Mor Deythan, they all had access to an arsenal of specialized weapons (including Volkite Chargers, Combi-Weapons, Missile Launchers with Suspensor Webs and Shroud Bombs). Today nobody knows if the Raven Guard, or their successors, have these warriors still in service.