Earth creatures

"Nothing sadder than an empty crab pot. Leeches ate most of the crabs."

- The Fisherman

Apart from the Xen, Race X and Synth creatures, Antlions or Combine Advisors, the Half-Life series features Earth creatures as well, all inoffensive.

Birds
Birds can be heard outside of Black Mesa. Sound file names suggest they consist of hawks and quails.

Penguins
In the non-canonical CTF mode of Opposing Force, Snarks are replaced by penguins with a grenade attached to their backs, serving the same purpose.

Insects
Insects can be heard in the exterior maps and some inside maps. Sound file names suggest they consist of flies, bees, and crickets.

Cockroaches
The Cockroach (monster_cockroach) is seen crawling in many places across the Black Mesa Research Facility. Its health is very low, since walking on it will kill it instantly. It is unknown if the creature is from Xen or Earth, but as it is seen in the Half-Life Hazard Course set before the Black Mesa Incident (at that point any creature located in the facility was in captivity), it is likely from Earth. Furthermore, the updated Decay model is identical to a real cockroach.

Rats

 * Rats (monster_rat) can be seen walking slowly and standing on its hind legs in the Black Mesa Hazard Course, in the Target Range. There, they do not react to the player's actions, and can only be killed using grenades.
 * They do not appear in Half-Life: Source.
 * Concept art suggests they were to be seen with scientists in the Black Mesa labs.

Barnacles
Barnacles (or Cirripedia, not to be mistaken with the Xen Barnacles) can be found on the rocks and the seafloor at the very start of Half-Life 2: Lost Coast.

Birds

 * With the humans, birds are the only Earth creatures seen during the Combine occupation. No explanation of this particular survival is given in-game.


 * As most NPCs, birds can be killed. Civil Protection officers and Overwatch Snipers can be seen shooting them from time to time, and Xen creatures attack them occasionally as well.


 * There are three known types of birds in Half-Life 2 and its episodes: the pigeon, the crow and the seagull, although many other types of birds (such as woodpeckers or miscellaneous birds of prey) can be heard in the Outlands.

Crows

 * Crows (npc_crow) are found in City 17, Ravenholm and the Outlands.


 * During the teleportation mishap in the Half-Life 2 chapter "A Red Letter Day", Gordon is first sent into the Wasteland, which is filled with crows, one of which gets caught in midair by Lamarr. Immediately afterward, when Gordon temporarily reappears in Kleiner's Lab, it appears that a crow came back with him, as one can be seen at his feet.


 * After Dog is taken away by a Dropship at the start of the Half-Life 2 chapter Anticitizen One, Combine Walls start moving towards Gordon and some very discordant music starts playing at the same time in the ruined tenement building nearby. Gordon proceeds to the building and arrives in the same corridor featured at the very start of the map, where he discovers the source of the music: an unplugged television on a pile of rubble blocking the doorway leading outside.  The television shows an image of the G-Man standing in a wooden boat with a crow on his shoulder. The television will shut off right after the G-Man straightens his tie, and the screen will break by itself when the player gets closer. A similar surreal scene can be seen in the WC mappack map " ", involving the G-Man being held in mid-air by a flying crow and an Ichthyosaur excreting gibs while flying, which may be a test map for that sequence.


 * False alarms are often triggered in White Forest's secondary silo by nesting crows.


 * In the Half-Life 2 chapter Route Kanal, the player can see a crow quickly take off, only to be eaten by a Barnacle.


 * Crows are sometimes seen feasting on corpses.

Doves
Although no dove is seen in-game, one is featured on one of the Combine propaganda posters scattered around City 17, above Combine logos, as an ironic symbol of peace. What appears to be a dove is also featured on a beheaded statue seen in Ravenholm's churchyard.

Pigeons
Pigeons (npc_pigeon) are found in the City 17 area and the Canals.

Seagulls

 * Seagulls (npc_seagull) are found along the Coast and over City 17.


 * The Fisherman from Lost Coast is very defensive of what remains of Earth's original species. His comments on these species gives the player more knowledge on the effects of Xen lifeforms and the Combine on Earth's natural environment. If the player shoots a seagull, the Fisherman will show he disagrees, saying "Hey, now! We've lost enough species already!", "What'd that gull ever do to you?", or "Don't shoot the gulls! They keep down the leech population!", implying seagulls eat Leeches.


 * Other unidentified birds (simply named "coast bird" in the sound files) can also be heard along the Coast.

Cats

 * Although not a single cat is seen in Half-Life 2 and its episodes, one is mentioned at the start of Half-Life 2: Kleiner apparently tested his resurrected teleport on a cat, aided by Barney, and the experiment went bad enough to give Barney nightmares about it.


 * When Barney learns the teleport is fixed, he replies "You mean it's working? For real this time? Because... I still have nightmares about that cat." Alyx is puzzled and asks several times "What cat?" but does not get an answer. Before being teleported, Alyx asks again "About that cat..." but again is not answered. Later in the game, Barney mistakes a distant Strider roar to a cat meow, saying "Did you hear a cat just now?", showing how traumatized he was by the mishap.


 * Several clues about the cat's fate can be found:
 * In Kleiner's Lab is a small teleport with a small cactus on it. If the player teleports the plant a few times, the teleport breaks down, making the plant disintegrate. An Orange Box Achievement, "What cat?", requires to break this mini teleport. This suggests the cat was apparently disintegrated.
 * In the playable Half-Life 2 Beta sound files, what actually happened to the cat is revealed by Barney during the original teleportation sequence: he states he had to turn the animal right side out again to make sure it was a cat.


 * The workshop style of Kleiner's Lab reminds of Seth Brundle's apartment/lab in the 1986 film The Fly, also featuring teleportation mishaps. Furthermore, the original version of the film, released in 1958, also features a mishap with a cat, when it is used as the first living being to be tested by André Delambre. Half-Life 2's cat mishap is probably a reference to that scene, although in the film the cat does not reappear and is lost somewhere between the two teleports, while the baboon replacing it in the remake fits better Barney's remark in the original teleportation sequence, the animal being turned into a dying, deformed monster.

Crabs
In Lost Coast, if the player grabs a crab pot with the Gravity Gun, the Fisherman says that most of the crabs have been eaten by Leeches. That implies some could still be alive, even though the ocean has mostly been destroyed, and that the Fisherman may even catch them.

Frogs
Frogs can be heard but not seen in the Canals.

Horses

 * During the City 17 uprising, a Citizen says he could "eat a horse, hooves and all". Whether horses can still be found is unknown.


 * A bronze horse statue can also be seen on a column at the end of the chapter "Follow Freeman!", in the map " ". It was originally featured in WC mappack maps such as " ", " ", as two horses, each on a column, or in the museum in Palace. It can also be found alone on a column in " ", the first street map following Palace. The prefab map " " also contains two horse statues.

Insects
Insects can be heard but not seen in the Canals, in the Outlands, along the Coast, and outside Aperture Laboratories. Flies can also often be heard near corpses.

Rats

 * Rats can be heard in Ravenholm and the City 17 Underground, but are not seen.
 * An incomplete rat model can be found in the Half-Life 2 Beta files, suggesting rats were actually to be seen.

Whales

 * Along the Coast in the playable Half-Life 2 Beta only, whale sounds can be heard underwater. As seen in the sound files, however, they actually are Ichthyosaur sounds, named "ping1" and "ping2".


 * The seafloor was originally to be full of beached ships and whale bones, recalling the real world Aral Sea.

Worms
Worms can be seen on the chest of the rotting Citizen corpse model.

Birds

 * Birds can be heard outside at the very end of the game.
 * Some birds such as a goose, a crow or a sparrow can also be seen on GLaDOS' screens at the end of the game.

Chicken
As seen on a clipboard found in the Enrichment Center in Portal, Test Subject #042 was apparently a chicken that failed the tests it took. It is unknown if these tests involved the ASHPD as they are never mentioned elsewhere, and no dead chicken(s) can be found across the Enrichment Center.

Bees
In a memo seemingly by Cave Johnson revealed during the Portal ARG, he states that the "bean counters", a higher authority than himself, told him it was against regulations to practice beekeeping in his office. Johnson ignored this, and his bees' honey won first place in the state finals.

Crickets
Crickets can be heard outside at the very end of the game, and are named as such in the captions.

Mammals
Some mammals can be seen on GLaDOS' screens at the end of the game. They include cows, a horse, a black bear cub (original image), a bunny, goats, and sheep.

Miscellaneous
Form 21 of the Enrichment Center Test Subject Application Process found on ApertureScience.com contains a list of 1345 items mostly consisting of animals, taken from an index found at the website AnimalPicturesArchives.

List of appearances

 * Half-Life: Day One
 * Half-Life
 * Half-Life: Uplink
 * Half-Life: Opposing Force
 * Half-Life: Blue Shift
 * Half-Life: Decay
 * Half-Life 2
 * Half-Life 2: Raising the Bar
 * Half-Life 2: Lost Coast
 * Half-Life 2: Episode One
 * ApertureScience.com
 * Portal
 * Half-Life 2: Episode Two