Combine Stalker

Stalkers are humans who have been drastically altered, both physically and mentally, through extreme,

brutal Synth engineering by the Combine. They are servants, mindlessly operating machinery and guarding the core in the Citadel.

Overview
According to Ted Backman, the enemy's designer, the Stalker was inspired by a black version of the Half-Life Skeleton multiplayer model. The developer who created and featured it in his levels proposed that a "black skeletal character that can hide in dark shadows and leap out at you as you got close" be included in Half-Life 2. The horror aspects of this concept were then further developed.
 * “The Stalker idea came from wanting something that crept around in the shadows and then lunged at you. We took a half-baked idea and turned it into something more horrific because the Stalkers are really the victims. When you face them, they are these crazed half-human things that you can't help but almost pity. I was hoping to put enough humanity into the things that it wasn't just a scary monster. Instead it was something that presented a moral dilemma every time you had to deal with it, which I think is a more interesting problem. It is more horrific to have to deal with an insane hostage than something that just wants to eat your brains.”
 * — Ted Backman

Marc Laidlaw expanded on the enemy's origins, remarking that Stalkers were one of the earliest creatures put into Half-Life 2, and the developers tried them in several gameplay environments. Initially, they were foes that crouched down in the dark and followed players from behind, surprising them when they turned around. While Valve had numerous gameplay concepts for them, such as Stalkers being used in puzzles where they could cut through sheet metal with their beams, they ultimately didn't end up being fun. As these sequences were "scripted and stagey," they decided to aim for an emotional impact instead and make Stalkers an important story element.

The first known in-game iteration of the Stalker, found in the Half-Life 2 leak, was based directly on the concept art featured in Half-Life 2: Raising the Bar. Captions included in the image provide some in-universe backstory to this cut design. Appearing even more skeleton-like than in the final version, the Stalker can be seen wearing not only a blinder but also a muzzle through which it is able to lap up fluids; both devices are said to have been installed for ease of control and handling. In addition to having portions of its forearms and lower legs severed, the human captive is hobbled during the conditioning process to further prevent escape attempts. Once it is ready to serve, braces that allow for walking are fixed to the stumps. This design was then expanded upon, equipping the Stalker with tools attached to each of its arms, resulting in the final iteration included in the Half-Life 2 leak.

Stalkers were originally planned to be featured frequently as hostiles throughout Half-Life 2, being notably fought in the Air Exchange, on the Borealis, and in City 17, among other locations. Stalkers were apparently being transported on the Borealis, one being found inside a hanging cage. A large group of them were also intended to be encountered in Kraken Base where they assisted Elena Mossman in running the underwater research base. The combat elements were ultimately cut and, later, re-introduced in Episode One. In the Half-Life 2 leak, Stalkers are much more agile, have melee attacks, a more damaging laser, and emit wilder shrieks and babbling when active.

On August 16, 2016, the source files of a Stalker model that appears to be earlier than the one found in the leak were publicly leaked. It is slightly taller and doesn't include the arm-attached tools. Of note are previously unseen animations, notably   and   which convey a much more cowering personality.