Sentry Turret
The turret consists of a powerful machine gun built onto a tripod. The gun itself rests on a turntable mechanism of sorts on the top of the tripod, which allows it to rotate and fire in any direction, unlike the equivalent device in Half-Life 2, which has a narrow field of fire.
Tank
The M1A1 Abrams is a US military main battle tank used by the HECU during the Black Mesa Incident.
Headcrab
Headcrabs are fairly small creatures, consisting of a rounded, tan body with four reddish legs for movement and a pair of larger frontal claws for attacking prey or as additional support when standing still. Under the headcrab's body is a large rounded mouth surrounded by pointed flesh (which also includes the headcrab's stubby front legs) and a sharp, claw-like "beak.
Zombie
Zombies do not respond to fire and will walk blindly into it, though they feel it on them and notice they are burning once they are on fire. Zombies mainly attack with swipes from their claws, but in Half-Life 2, they gain the ability to forcefully punt objects towards the player.
Houndeye
Possessing only three legs and a thorax, the Houndeye is a faint yellow-green in color, with electric blue tiger-like stripes adorning its spine. In place of a head there is a large, black, compound eye protected by eyelids. Houndeyes apparently communicate through a series of high-pitched sounds somewhat similar to the barking of dogs, on which its behavior and movement are also partially based, hence its name.
Bullsquid
Bullsquid are able to survive, if not thrive, in environments that are unfriendly or even toxic to humans, including sewers and pools of radioactive, chemical, or biological waste
Barnacle
Barnacles are relatively weak; a few shots from a 9mm pistol will kill them. They are most easily dispatched from a distance, although players who become ensnared are usually able to kill them before being consumed. The barnacles are weak against the crowbar, which in both games, kills them in a single hit.
Vortigaunt
Vortigaunts have a mottled green-brown skin; similar to a reptile's, sharp teeth, and clawed hands. Vortigaunts have a slightly hunched posture, and their faces are dominated by a large, single red eye, surrounded by five smaller eyes.
Alien Grunt
The Alien Grunt is the primary combatant of the Nihilanth's Xen army. Tough, strong, and extremely hostile, Alien Grunts stand about seven feet tall and are armed with a biological weapon called the Hivehand, which contains Hornets, insect-like flying creatures that seek out living targets and attack them.
Alien Controller
The Controllers share many characteristics with the other Xen creatures, especially the Nihilanth, suggesting that they are a related species.