Earth creatures

Good

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

- The Fisherman

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

Birds
Birds can be heard in the exterior maps. Sound file names suggest they consist of hawks and quails.

Cockroaches
The Cockroach (monster_cockroach) is seen crawling in many places across the Black Mesa Research Facility. Its health is very low, since it only takes to walk on it to squash it. It is unsure if the creature is from Xen or Earth, but as it is seen in the Half-Life Hazard Course, it is likely from Earth.

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

Rats

 * Rats (monster_rat) can be seen walking slowly and standing on its rear legs in the Half-Life Hazard Course, in the Target Range. There, they do not react at the player's actions and cannot be killed in the original, but are mortal in the Blue Shift.
 * Concept art suggest they were to be seen with scientists in Black Mesa labs.

Birds

 * With the humans, birds are the only seen Earth creatures 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.

Crows

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


 * At the start of the Half-Life 2 chapter Anticitizen One, after Dog is taken away by a Dropship, Gordon proceeds to a building and arrives in the same corridor featured at the very start of the map . Instead of the door leading outside is rubble where can be seen the source of the music: an unplugged television, showing an image of the G-Man standing in a wooden boat with a crow on his shoulder. The television will shut off right after G-Man straightens his tie, and the screen will break by itself when the player gets closer.


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

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 are still alive.

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

Horse
During the City 17 street battles, a Citizen says he could "eat a horse, hooves and all". Wether horses can still be found is unknown. A horse statue can also be seen at the end of the chapter "Follow Freeman!". It was originally featured in WC mappack maps such as " " or the Skyscraper, both versions.

Insects
Insects can be heard but not seen in the Canals and the Outlands. 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 (but not in the final game), whales can be heard underwater.
 * The seafloor was originally to be full of beached ships and whale bones, recalling the real world Aral Sea.