From Halopedia, the Halo wiki
Captains are one of the highest castes of the Yanme'e, and rarely encountered on the battlefield. They can be denoted by their red-coloured carapace and - unusual for Yanme'e - energy shielding. They are also often equipped with plasma rifles (or for those individuals serving under the Jiralhanae, Brute plasma rifles), rather than the plasma pistols and needlers afforded to lower-ranked individuals.
Halo 3: ODST
Yanme'e Captains are rarely encountered in Halo 3: ODST. In Firefight, Captains can spawn alongside Yanme'e infantry (consisting of Minors, Majors and Ultras) but do not replace infantry - instead counting as "Yanme'e Leaders" alongside the gold-clad Captain Majors. Captains and Captain Majors spawn in a ratio of 3:1.
List of appearances