According to X-49, 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 he discontinued his line of work killing for Aku and went into retirement. The key factor in differentiating X-49 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 "Episode L: Tale of X49."
The X-Models were eventually replaced with various "drone" robots (such as Aku's Beetle Drones), despite the latter series of robots being highly ineffective against Jack on most occasions where he fought them.
They were likely erased from existence after the past Aku's destruction.
The 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.