As far as "hereditary socially prestigious cavalry warriors" go, there were two primary groups in the Ottoman Empire that fit the bill, distinguished by their mode of remuneration: cavalrymen paid in grants of agricultural revenue (timars, from which the name "timariot cavalry"), and cavalrymen paid in cash salaries from the sultan's imperial treasury. The latter were thus, like the Janissaries, technically the household slaves of the sultan ("kapıkulu"), and therefore I'll refer to them from now on as kapıkulu cavalry, although the Ottomans most commonly referred to them as the Altı Bölük Halkı, or "People of the Six Regiments." Members of both groups were known as sipahis ("cavalrymen"), which often misleads even some professional historians into imagining that they were one and the same, but the distinction was important, as I'll explain shortly. Both kinds of cavalrymen were regarded as part of the Ottoman elite: in Ottoman political theory society was divided into two groups, the askeri ("military") and the reaya ("flock"). The latter were the taxpayers, and the former defined by having been granted the privilege, by the sultan's permission, to receive some portion of those taxes. Therefore it was technically the sultan's generosity, and not actual hereditary privilege, that made these men part of the social elite. Despite this conceptual link between elite status and sultanic beneficence, there existed a general consensus that the sons of members of the elite deserved at least some share of their fathers' status. The Ottomans didn't by and large think of their elite as possessing an inviolable hereditary right to "noble" status, but membership in the elite was hereditary nonetheless, so long as those elites didn't take any action that would compromise that status.

The timariot cavalry were a diverse bunch. Most were either the descendants of warriors who had been given timars as a reward, or were the descendants of pre-Ottoman nobility who had been assimilated into the timariot class (Christian timariots fought in the Ottoman army even in the late fifteenth century, but were gradually decreasing in number as they converted to Islam). The so-called "timar system" that was extended across the empire in the fifteenth century reserved a large proportion of the empire's agricultural revenues for their upkeep, with each timariot inheriting the right to hold a timar somewhere in the empire, rather than in a specific location. Thus the timariots could not tie themselves to a single location on a hereditary basis, but each generation would be moved from place to place - in theory. In practice, the son of a timariot was sometimes able to inherit not only his father's place in the system, but his specific timar.

By the late sixteenth century, economic and technological change had lowered the military value of the timariot class, who by and large fought with traditional weapons - bow, lance, and sword, and were poorly suited to battlefields now filled with gunpowder-wielding infantry, and to a style of warfare that demanded more competence in sieges than in field battles. Consequently, the economic basis of the timariot class was increasingly undercut, as the central government allocated a larger percentage of the empire's tax revenues for the upkeep of the kapıkulu standing army and to fortress garrisons rather than leaving them to the timariots. The number of cavalrymen supported by the timariot system (including not only timariots proper but also their armed retainers) decreased from its mid-sixteenth-century height of roughly 1-200,000 to around 70,000 by the first decade of the seventeenth century and continued to shrink thereafter, although historians disagree on the exact magnitude of this shift. Timariots' role in the army changed from that of light cavalry to that of an auxiliary logistical force, performing tasks such as transporting cannon and constructing fortifications. By the mid-seventeenth century, most timariots were opting to pay a cash tax to the central government rather than go to war.

As the timariot class underwent this transformation, the kapıkulu cavalry experienced a transformation of their own. First, the size of the organization greatly expanded: from 5,000 in the early sixteenth century, to 8,000 by the 1560s, to 20,000 by the first decade of the 17th century. A number of historians have expressed surprise about this, because technological change was supposedly making cavalry obsolete, but it seems to me that the kapıkulu cavalry were filling the gap created by the decline of the timariots. After all, the Ottomans still required cavalry - but cavalry better able to adapt to the conditions of 17th-century warfare. For example, while timariots expected to be permitted to return home to their fiefs at the end of each campaign season, the kapıkulu could be more freely ordered about, and kept in the field or attached to fortresses for long periods of time. Whereas volunteer warriors could have expected to earn a timar in the fifteenth and sixteenth centuries, in the seventeenth it was the kapıkulu cavalry that was absorbing the better part of worthy new recruits.

So who were the kapıkulu cavalry? In the sixteenth century the organization consisted largely of devşirme recruits, i.e. men who had been taken from their Christian homes in their youth and trained to be servants of the sultan. Many kapıkulu cavalryman had originally been Janissaries, who had been selected for their merit for promotion into the more prestigious and better-paid cavalry forces. Others were the sons of bureaucrats, administrators, and high officials, who used their political connections to get their offspring into the organization. In the seventeenth century, as I mentioned, the cavalry became more open to volunteers who could prove themselves in war.

Membership in the kapıkulu cavalry became especially attractive in the early seventeenth century because of the expansion of privileges awarded to veterans. Long-serving cavalrymen were granted the opportunity to become tax collectors, providing them with a form of non-combat employment and allowing them to enrich themselves. While they had once resided only in Istanbul and its environs, in the seventeenth century wealthy kapıkulu cavalrymen were ubiquitous in the cities of Anatolia and the Balkans, constituting some of the most influential members of the local elite. As a consequence, the organization morphed from being a relatively straightforward military force into an influential social group, with an economic reach stretching across much of the empire's core territories. If you were a taxpayer in Anatolia or the Balkans in the early seventeenth century, chances are the figure who came to collect from you would be a kapıkulu cavalryman or his agent. The power of the organization, and the propensity of charismatic figures from within it to launch rebellions when dissatisfied with the government, led to a backlash from the empire's administrators. The aforementioned privileged access to tax collection positions was eliminated in 1632 (although they could still compete for tax farming contracts normally), and in 1658 the organization was thoroughly purged of "troublemakers," and reduced in size by about a quarter. After this point it is generally assumed to have reverted to a relatively well-disciplined military organization again, although historians have not investigated this later period so closely.

Much has been written on the timariots, the best introduction being Halil İnalcık's The Ottoman Empire: The Classical Age, 1300-1600 (1973), a book that is out of date in some respects, but is nevertheless still a good introduction to Ottoman institutional history. More recent is Colin Imber, The Ottoman Empire: The Structure of Power, 1300-1650 (Second edition 2009). On the kapıkulu cavalry there is very little in English, unfortunately, but I could answer any followup questions.