The Sergeant Sturdy Common Attack Droid was the first combat automaton produced and approved by Equestria for front line use. The Saddle Arabian Sultanate had been using combat robots for years, but it wasn't until they became sufficiently dangerous, and casualties became great enough to warrant parallel development. They advance with single-minded determination and show only cursory regard for damage. Each is fitted with a unique weapons load-out based on their mission.
Many are still active in Second Era facilities, and are almost uniformly hostile. Lozenge and company are forced to shoot their way through multiple encounters with Sergeant Sturdy models.
Source: Moral Mathematics
Source: Dirty Hearts
Source: In Oculo Venenum