Some people, including the Horseclans' founder and mentor-figure, Milo Morai, were "Undying"—effectively immortal or at least unaging and almost impossible to kill by means other than suffocation. The Undying were also sterile, which was a source of anguish for some of them.
The known Undying were
In addition, Bookerman mentions two other Undying, one of whom he claims was dead.
Although not among the Undying, two other major characters are extremely long-lived, Hari Krooguh being almost two hundred years old when last mentioned in the books and Bili Morguhn, aka Bili the Axe, remaining active until his death from bear-inflicted injuries at the age of almost a century. In addition, both Hari and Bili possess extraordinarily strong mental powers of telekinesis and illusion (as does Aldora).
It could be argued that although never categorized by a name they form a third type of humanity.
The Horseclans looked down on "dirtmen", or farmers, and by extension all non-nomads, although they later resettled on the east coast. Many "Dirtman" communities were descended from odd, out-of-the-mainstream groups of Americans, and often had religious beliefs that were unpleasant at best.
The main civilization in North America during most of the books is that of the "Ehleens",—a conglomeration of Greek-speaking Mediterranean peoples who had invaded the eastern half of North America and set up kingdoms. The main Ehleen kingdom, Kenooryos Ehlas (New Greece), was dominated by a thoroughly corrupt, debased version of the Greek Orthodox Church, and the conduct of whose rulers is, in some ways, also reminiscent of Hellenistic kings.
Other peoples the Horseclans dealt with were "Ganiks", "Mehrikans", and "Ahrmenee". The "Ganiks" were degraded descendants of hippie communes, whose religion of "Orghanikonservashun" (organic-conservation) forbade them to eat animal meat (but not human meat; Ganiks were enthusiastic cannibals), bathe, or hot-work metal (due to the pollution caused by burning coal).
"Mehrikans" were descendants of present-day Americans. "Ahrmenee" were descendants of present-day Armenians living in the Appalachian Mountains.
In a similar fashion to the world of Robert E Howard's Conan and of Jack London novels, civilisation is seen as having a softening and corrupting influence in comparison to the hardy Ahrmenee and the Horseclans, although not to the extent that they outweight the peace and prosperity that Milo and others work towards.
In addition to the novels, there are two short story anthologies containing stories by both Robert Adams and others set in the Horseclans setting, Friends of the Horseclans, (1987) and Friends of the Horseclans 2 (1989)
The GURPS system had a worldbook (GURPS Horseclans) and a solo adventure, (GURPS Bili the Axe - Up Harzburk!), devoted to the series.