The Jackal (Kig-Yar) and Skirmisher are of the same species, with the larger, more heavily muscled specimens comprising grenadier-type units (Skirmishers) and the smaller, more lightly built specimens seeing deployment as dedicated ranger or sniper units (Jackals). They are second bottom of the Covenant cast system, with only the Grunts being lower than them.
In 1342 CE, the Covenant encountered the Kig-Yar (or "Jackals"), a mercantile race of pirates and cutthroats. Handy fighters, the Kig-Yar are extremely pragmatic and independent-minded. These reptilian egg-layers have matriarchal lines that have controlled the skies of their home system around Eayn for centuries.
When Confronted with the prospect of a superior force bearing down upon them, the Kig-Yar chose to convert and profit the immediate expansion of trade, and therefore wealth, that the Covenant could provide. The Kig-Yar soon became useful explorers and traders (and often thieves and smugglers) in the Covenant fleets.