Promethean Crawler

Promethean Crawlers are a unit of advanced form of artificial intelligence created by the ancient Forerunner race alongside other models of Promethean including Knights and Watchers that appear in the Halo Universe.

Physical Appearance
Crawlers have a very distinct appearance, seeming to look very similar to various species of small, four-legged animals. They have a very strong armor plating, similar to the Crawler's fellow Prometheans, but seem to lack heavy armor on their forehead, as they can be easily destroyed with a headshot. Early units of Crawlers, as seen on the Forerunner planet, Requiem, where silver in color, but later variants of the standard Crawler, like the ones used in Cortana's Created, have a more dark grey and almost black color.

History
Promethean Crawlers have been used by the Forerunner race for years, being created during the final years of the Forerunner-Flood War and being placed on key Forerunner worlds to aid in the fight against the Flood infection. The most notable use of Promethean Crawlers was when the Forerunner known as The Liberian placed several Crawlers, along with other Prometheans, on the artificial planet, Requiem, to guard the rogue and insane Forerunner, known as the Ur-Didact. These Crawlers would later be encountered by the human soldier, Master Chief, who ended up accidentally releasing the Ur-Didect from his prison and used his powers to take control of all the Prometheans on Requiem, including all of the Crawlers. Later, after the Ur-Didact's death at the hands of Master Chief and his human-created A.I., Cortana, all of the Crawlers under the Didact's control were put into use by the Sangheili, as soldiers in Jul 'Mdama's Covenant remnant for a brief period of time until they came under the control of Cortana and her Created, rebelling against the Covenant to join her.

Alpha Crawlers
Promethean Alpha Crawlers, also known as Prime Crawlers, are a variant of Promethean Crawler that are designed to act as the leader of a designated group of Crawlers. They can be easily identified by the spikes on their back and unique head design. Alpha Crawlers are usually seen in smaller amounts than standard Crawlers. Similar to standard Crawlers, most Alpha Crawlers were silver in color, but later units were seen with a white tint instead. It is possible that this color change from silver to white may have been to better identify the Alpha Crawlers from the standard Crawlers and Snipers.

Sniper Crawlers
Promethean Sniper Crawlers are another variant of Promethean Crawler that, as their name suggests, are equipped with a Z-750 Special Application Sniper Rifle and are programmed to attack threats from a distance. Similar to the Alpha Crawler, Snipers are not commonly used in combat but have been proven to be deadly adversaries nonetheless.

Appearances

 * Halo 4 (first appearance)
 * Halo: Spartan Strike
 * Halo 5: Guardians