From Halopedia, the Halo wiki
Dragoons were a form of first-order Forerunner navy warships. Used from the earliest interstellar expansion by the Forerunners, dragoons were accompanied by vast entourages of weapon-ships and combat Sentinels.
A notable class of dragoon was the Sojourner-class dreadnought, also described as a prime dragoon for its pinnacle Forerunner technologies and effectiveness. It was introduced shortly before containment efforts against the Flood began, and continued to fight in all major engagments of the Flood war.