Antlions are extra-dimensional alien insectoids deposited on Earth from Xen by portal storms that occurred in the aftermath of the death of the Nihilanth. The smallest of these ferocious bugs are the size of small dog, and the largest are bigger than a tanker truck. These creatures likely originated on the Vortigaunt's home world and lived in subterranean colonies. Antlions do not have eyes and so rely on their sense of smell and vibration.
These are by far the most common variety of Antlions. They are beige-green in colour, with elongated quadrelateral-shaped backs, four pointed legs and triangular heads. This species of Antlion live in packs and attacks aggressively, ignoring everything in their path and often lunge at opponents and using their legs and teeth to scratch at them. In some areas of the Half-Life games these Antlions will spawn infinitely unless certain areas are avoided or their spawn point is otherwise blocked. However Common Antlions can be easily killed and even temporarily flipped over by the Gravity Gun. Later on in Half Life 2, Gordon Freeman receieves the ability to control a pack of antlions that can attack enemies for him.
Antlion guards appear only a few times throughout the Half-Life games, in large, open areas. They are beige and red or light blue in colour, have four legs and large top sections. Antlion guards usually attack alongside groups of Antlions who they command via pheromones emitted from the 'Pherapods' on their bodies and attack by charging aggressively, in a straight line. Antlion Guards are much harder to kill than Common Antlions and often require the use of powerful weaponry or lengthy battles.
These Antlions have only appeared in underground areas, early in Half-Life 2: Episode Two. They are creamy-yellow coloured and have termite-like bodies which hang close to the ground, with the knees of their legs extending above the height of their body. Antlion Workers attack with their claws or by spitting neurotoxic acid at their enemies. They are roughly as difficult to kill as Common Antlions and can also be temporarily flipped by the Gravity Gun, but upon death they will explode, sometimes catching their attackers in the deadly acid that sprays up.
These are Antlion larvae, only seen in the underground sections in Half-Life 2: Episode Two in which the Antlion Workers appear. These grubs are light yellow in colour with brown mouths and are luminous. They cannot attack and when touched they wukk dies, releasing a yellow fluid which provide a small amount of health and is used by Vortigaunts for healing.
An Antlion Guardian is encountered near the end of the subterranean area in Half-Life 2: Episode Two. They look identical to the Antlion Guards but with darker beige skin and some areas of them that glow green. They take the role of protecting nests of Antlion Grubs and will strongly pursue intruders in the nest, using ramming attacks and sometimes acid against them. Much like other Antlion guards these creatures are difficult to kill and often require the player to keep their distance.
In the PC version of the action/racing game Renegade Ops, Gordon Freeman is a playable character. His special ability is to unleash a couple of Antlions that will act on their own and seek out enemies to kill. Their damage output, their health and even the amount spawned can all be altered depending on which perks the player chooses.