From Silver Sun Republic Codex

In ancient times of the Republic there were no methods of stabling or providing great numbers of horses by the state for its militaries and only certain families owned several horses. In times of war the nobles would negotiate with these families granting them additional lands and a higher social status to ensure their armies were equipped with enough mounts. As war became more commonplace these families enjoyed an elevated measure of wealth rising above their peers since horses were so valuable. Over time this gave way to a new caste as the families reinvested their wealth to become a self made caste below the nobles in social status, but above their peers economically. Eques means horse in the old tongue and has transformed into a self risen caste between Plebeian and Nobilitas.

The Republic further solidified the caste by allowing members to run for and hold public office. Many Nobilitas are Eques who have successfully won elections and created their own political dynasties. There are not many true Nobilitas who can trace their roots to the ancient families and bloodlines any longer. To ensure the continuation of the social elite, the older families allowed the Eques to elevate themselves to gain allies against the Plebeian and hold on to their status. Some Eques enjoy so much wealth and economic success that they are able to forgo the elections and elevate themselves to Nobilitas through finances alone.