- In Half-Life 2, the Claw Scanners appear in a couple of chapters and don't appear again until late in Episode 1.
- Toward the end of the game, there is also one Overwatch Elite who uses a submachine gun, the only one in the game to do so (all others carry Pulse Rifles).
- In the original Half-Life, the Black Ops assassins only appear in two rooms in the entire game, and even then only in groups of three. In Opposing Force, they're more common and have more variations, but only slightly so.
- Only one of the male Black Ops grunts uses a sniper rifle. All others use the M4/MP5.
- The alien auto-cannon. One appears in one corridor in the original game, having been set up by Alien Grunts, and nowhere else.
- While common in the last levels of the original game, the Alien Controllers only appear once in Opposing Force.
- Also in Opposing Force, there is exactly one Black Ops Apache.
- Two Bradley IFVs appear in the Half-Life chapter "Surface Tension", one of which has a TOW missile launcher while the other doesn't. They don't show up again.
- The Civil Protection officers were a common enemy during the early chapters of Half-Life 2. In Episode One however, just a single squad of them appear during the last wave of the Escort Mission at the end of the game; they're then completely absent from Episode Two. Episode One also uses the Gunship and Strider for one-off boss battles, both of which were more numerous in Half-Life 2 proper. In addition, only one Antlion Guard is fought in Episode One. Plus one Combine APC.
- The Antlion Guardian in Episode Two. Essentially a Palette Swap of the normal Antlion Guard (but otherwise identical), it's only encountered once in the entire game.
- The Fast Zombie Torso, also in Episode Two, appears a handful of times throughout the game. Fast zombies normally can't survive their legs being ripped off, unlike their standard cousins.
- The much-maligned fangame Hunt Down The Freeman has a supposed Antepiece involving a medkit thief getting eaten by a barnacle near the start of the game. The rest of the game has no other barnacles.
- In Half-Life, only three ceiling turrets appear in the game. Two of them are encountered in "Office Complex", and the third one, much larger than the first two, is encountered near the start of "Forget About Freeman".
- In Half-Life 2 and Episode One, Manhacks were a common enemy, but in Episode Two, they are only encountered once, when the Combine breach White Forest.
- The tanks encountered in Surface Tension on the first game count as there's only a few and no two are alike, some have turrets with a very limited rotation range making you safe as long as you are behind them, while others can turn a full 360. Some shoot rockets while others have some sort of Hitscan mortar type weapon. Finally, while most have only one gun there's one with a machine gun type weapon in addition to the main (rocket) turret.
- In Half-Life, Xen has their own version of ceiling turrets that appear on the large island that the player has to jump to in the "Xen" level. This island is the only location where the ceiling turrets appear.
Unique Enemy / Half-Life