Although many of the enemies from Half-Life have returned, many don't, while some make only cameo appearances. There are, of course, plenty of new foes to face off against; check this section for a description of each.
The large man-traps known only as Barnacles are a repeat enemy from the first game, and as in Half-Life, you shouldn't have any problems defeating them or getting around them. In fact, you should have an even easier time killing them in most instances, due to their new and improved behavior.
As in the first game, Barnacles will latch on to any organic entities that come into contact with their adhesive-coated tongues, and slowly draw their prey up into their maws. This includes Overwatch forces, for the most part, the occasional headcrab, and of course Gordon himself. You shouldn't have too many problems spotting the Barnacles from a distance, though, as their tongues are pretty easy to spot, save in darkened rooms. When you do notice a tongue, killing the beast is a simple matter of popping the Barnacle's main body with eight or nine shots from your pistol; they're immobile, so they can't dodge your fire.
A new tactic for bypassing Barnacles arises from Half-Life 2's new physics engine; now that you have the ability to manipulate items in the game world, you can trick the Barnacle into thinking that it has some tender prize by throwing an inorganic item into its tongue. A barrel or crate usually does the trick; the Barnacle will retract its tongue in the anticipation of a tasty treat, allowing you to pass underneath it without needing to waste any ammo. Explosive barrels do the trick quite nicely; if you wait until the barrel has nearly reached the Barnacle, then shoot it until it explodes, it'll kill off the Barnacle, as well as any other Barnacles nearby, making this an efficient way to destroy the Barnacle nests you'll sometimes encounter in the earliest portions of the game.
The headcrab was perhaps the most distinctive enemy in Half-Life, inspiring countless startled jumps as they would hop out of darkened hiding spots in an attempt to eat your brains. They were, of course, never successful, but being creatures of instinct, they never quite learned to leave you alone, and indeed, are back in Half-Life 2 to make your travels difficult. What's more, there are now three different kinds of headcrabs to deal with, each with their own special traits.
The old-school headcrab is much the same as it was in Half-Life; they'll creep along in search of prey, and when they get close enough, they'll leap and deal a bit of damage when they impact you. If you spot them while they're crawling, you can easily take them out with a few shots from your pistol, but they're rarely visible to you before they get into jumping range; they're more often positioned near a corner or the top of a ladder or in some other cramped location, where they'll get a free jump at you before you can react. You can sometimes get a bit of forewarning by listening for their distinctive muttering, but this is rarely audible before they've spotted you.
Luckily, headcrabs don't do too much damage, and can't take much, either, letting you kill them off with pistol shots or by shoving a crate at them with your gravity gun. They're most dangerous when encountered in groups of three or four, where they can corner you and prevent you from getting away by pressing on you from all sides; in these instances, you'll probably want to use either a shotgun or an SMG to quickly clear a path out of the maelstrom.
For lack of a better nomenclature, we'll call these guys quick headcrabs. They're similar in appearance to normal headcrabs, save for somewhat longer legs, and skitter rapidly about the floor, making them difficult to shoot. Besides their increased speed, though, they don't contain any other noticeable upgrades over their less-fleet counterparts; they deal roughly the same amount of damage and can be killed in the same manner, although in this case, you're going to want to go straight for the SMG or the shotgun, as they're difficult to hit with your pistol unless you can fire without fear of them attacking you.
Now, these guys are annoying. Toxic headcrabs are black, and a bit stouter than the normal variety, and a lot tougher. They sport the ability to take much more damage than the other varieties - on hard mode, you'll need two magnum rounds to do the job - but what's worse is their attack; when they hit you, they'll instantly reduce you to one health due to their potent neurotoxin. You'll regain all of the health you lost, eventually, but the damage is still real, and if you get hit by anything while you're weakened, you'll still die. Luckily, the toxic crabs are less agile than either of the other varieties, and move relatively slowly when they're not actively pursuing you, letting you pick them off from a distance. When they get the jump on you, though, you can quickly find yourself reaching for the quickload key, so try to find a nice quiet place to hide while you recover from the effects of the poison.