Depends on the period.
You have Napoleon's Imperial Guard, The Roman Praetorians (Although they were more just a Legion that was allowed to operate within Rome), The Ghurkha Regiment (Which has a presence in the Indian and British Armies)
Further back - the 10,000 immortals of the Persian army; The Sacred band of Thebes (Which was composed only of homosexual couples, under the idea that they would fight hardest for their "partner" and go into a beserker rage if said partner fell in battle)
Or the Praetorians' byzantine counterparts, the Varangian Guard. They were northerners, often pagan vikings, serving as the bodyguard of the byzantine emperor. With no blood ties to any of the locals and a strong code of honor they were considered...well, not incorruptible, because the byzantines were a fairly realist bunch, but very difficult to corrupt. Varangians, being strangers in a strange land, tended to stick together with a strong esprit de corps, they were undoubtedly elite combatants (to even be considered for a posting in the Guard you had to be among the best of a Proud Warrior Culture), they had their own traditions and rituals that set them apart from other armed forces of the empire and they cultivated one hell of a reputation over the centuries.
For a piece of backgrounf of one of my characters, I want him to be a former member of some extremely elite group, like 100 bullets\' minutemen. To that purpose, I thimk it\'d be interesting tocsee the real life inspiration for them, or groups that actually existed that share similar traits (ultra-elite, codes of conduct of their own, etc).
Has there actually been groups like that in the past?
Suffer not the witch to live.