According to X9, the X-series was much more adept than the preceding Beetle Drones in killing their quarry. It was only after every other X-Model was destroyed that X9 discontinued his line of work killing for Aku and went into retirement. The key factor in differentiating X9 from the others in the X-series was an experimental "emotion chip" that a quirky mad scientist installed in him, which eventually lead to his decommissioning at the hands of Jack in "Tale of X9."
The X-Models were eventually replaced with various "drone" robots (such as Aku's Beetls Drones) despite the latter series of robots being highly ineffective against Jack on most occasions where he fought them.
X-series robots are humanoid, with broad shoulders and skeleton-like features. They were usually equipped with mobster-style clothing and dual energy pistols. They had large, circular red eyes that glow.