Cryptid

Cryptids (from the Greek κρύπτω, krypto, meaning "hide" or "hidden") are a subset of animals and other creatures that are mentioned in local folklore and other accounts, but whose actual existence is questionable. This does not apply to anything that is widely acknowledged as existing. For example, the deathclaw was presumed to be a rumor or hoax in New California around 2161, but other regions of post-war America had many confirmed sightings beforehand.

Background
Cryptids were largely assumed to be superstition by the majority of the populace, with cryptozoologists, like Shelby O'Rourke, being taken as seriously as a child in a lab coat. But many cryptids actually existed before the Great War, and both the United States Armed Forces and United States Intelligence (like the Sugar Grove Sigint) kept records on any cryptid encounters up until the Great War. While considered to be hoaxes and superstition, cryptids still played a large part of the culture before the war as well, with examples of this cryptid craze coming from Point Pleasant erecting a Mothman statue, Vault-Tec University's football team called the Fighting Wendigos, Tales from the West Virginia Hills focusing on a different cryptid for each of its stories and Grafton holding a local holiday where the "Grafton Monster" would appear in the parade (and the real Grafton Monster appearing in it after the Great War).

After the Great War, cryptids began to appear far more often than they ever had before. This was noted by Shelby O'Rourke from her shack in The Mire where she theorized that the near extinction of the human race made room for cryptids to come out of hiding.

Aliens


Multiple species of aliens appear throughout the Fallout series. One race being the zetans from Fallout 3 that have appeared in each subsequent game.

Chupacabra
The chupacabra (Spanish for "goat-sucker") is a legendary creature of Puerto Rican origin. In Fallout: New Vegas, No-bark Noonan calls an invisible nightkin a chupacabra, after it has killed several brahmin around the town of Novac.

Ghost
The spirits of the dead that have been trapped in our earthly plane that had unfinished business or failed to attain something valuable to them. Not much is known about the function of ghosts, as the only examples of ghosts are Anna Winslow whose spirit became trapped in the Den after dying, kept on the mortal plane in agony due to the theft of her locket by a petty thief, and Lucy Grandchester who haunts the Grandchester Mystery Mansion for unknown reasons. Based on Anna Winslow's actions in her quest, after attaining the item they need most, the ghost will disappear with the item, leaving only a pile of bones.

Sasquatch


The sasquatch, though more commonly referred to as "bigfoot," is said to be a hairy, upright-walking, ape-like creature that dwells in the wilderness and leaves footprints. Depictions often portray them as a missing link between humans and human ancestors or other great apes. They are strongly associated with the Pacific Northwest (particularly Oregon, Washington and British Columbia), and individuals claim to see the creatures across North America. The sasquatch would continue to be a prevalent cryptid in popular culture until the start of the Great War, with some magazines even proclaiming titles such as Hardy as a Sasquatch.