Ages of Conversion

It is theorized that the Age of Conversion was the time that the Elites and Prophets began incorporating the other races into the Covenant, either through conquest or religious conversion. The Hunters resisted conquest more strongly than the other races, resulting in the creation of an Arbiter to help “tame” them (this Arbiter was killed). The Hunters defeated the Elites on the ground but were forced to surrender under threat of extinction through glassing. It is also possible that the Grunts were one of the species that was conquered, rather than converted. Almost nothing else is known about how the other races became part of the Covenant.

As the Covenant would have only picked 'worthy' species, it is likely that these beings would have, at some time in their history, either met the Forerunners, or evolved on a Forerunner planet.