Strider

 
Though the Strider is a creature not related to the Combine, it aides them in military  attacks against humanity due to some other means of persuasion the Combine use to control them. All of the armor, weapons and accessories on the Strider are Combine modifications that the Strider has evolved to work with seemingly. They are living creatures that were dominated and enslaved in their home world by the Combine but whatever free will they have left isn't quite clear 

Characteristics

The Strider is a half living, half synthetic war machine, standing several stories tall on its three beige armor-plated legs. These long stilt-like, jointed legs allow somewhat of a speedy movement and gives them the ability to crawl close to the ground and chase the enemy through tunnels even be carried around by Combine Dropships. The short black rods on the ends of each leg are used to pierce into and maneuver through challenging terrain. The main body, which is brownish-green in color, is focussed up at the top, looking almost helmet-shaped, with an appendage hanging down from underneath with a warp cannon attached to the tip. Its face is quite small and covered with a lot of gear including a pulse cannon and some sort of module that probably assists in sight or some other sense for locating and tracking targets. 
 

Attacks & Abilities

- The Strider is just as dangerous to be underneath as it is from a distance because the black spike on the ends of its legs cat as skewers and can kill or damage anything it comes in contact with. 
- The pulse cannon located on the center of the creature's face fires powerful but slow anti-personnel blasts down upon foot soldiers.
- The warp cannon, attached to the appendage on the creature abdomen, fires a slow charging but incredibly devastating beam of raw energy that can incinerate vehicles and even buildings. 
 

Tactics

- The most effective weapon against the Strider the Magnusson Device which can be attached to the Strider's hull via Gravity Gun and manually detonated via any long-range weapon, but since those don't appear until late in Episode 2, the best weapon is the RPG, which can take it down it 5 hits. 
- The Grenade Launcher (secondary fire on the MP7) and the secondary fire do cause damage but not nearly as much as the RPG. Usually Strider encounters involve ammo crates with infinite rockets so they wouldn't even have to be used for a last resort. 

Appearances

HALF-LIFE 2: 
Striders can be seen early on in the game in the Point Insertion chapter patrolling the streets of City 17 before Gordon is considered a high threat. They are not see again until much later in the game during the Anticitizen One chapter, where it makes its entrance firing its warp cannon into street fought on by the human resistance. The Striders are not actually fought until the Follow Freeman chapter where they defend the Overwatch Nexus near the base of the Citadel. They also make an appearance in the Our Benefactors chapter inside the Citadel once in battle and another time marching along the Citadel's corridors on Gordon's on-rails ride up to the top. 

HALF LIFE 2: EPISODE ONE:   
The Strider is only encounter twice in the second episode. The first is right after the Low Life chapter, as Gordon and Alyx Vance ascend from the zombie-infested undergrounds and up to the surface of City 17, which is now mostly destroyed seen by a vista with a Strider walking across its ruins. The Strider is seen only once more in the climactic battle at the end of the Exit 17 chapter in the train station, the last thing Gordon fights before escaping the city with Alyx.
 

HALF LIFE 2: EPISODE TWO: 
The Strider makes a few appearances before they are actually fought in the end game battle. The first being at the very beginning of the Freeman Pontifex chapter (after the Antlion caves), where they can be seen escorting Combine troops and Combine Advisor pods on a bridge above. The second instance is at the end of the Under The Radar chapter when a Strider appears from underneath a downed Dropship that fights with Dog when he appears out of nowhere. The most prevalent occurrence of the Strider happens at the end of the Our Mutual Friend chapter when White Forest is under attack by many Striders and an escort of Hunters as the climax of the game.