The monster who comes out every New Year and is repelled by red paper, firecrackers, and noise.
Documented
The customs are old, the beast is young
Red couplets, firecrackers, and the New Year vigil are documented for centuries. The monster Nian who fears exactly these things is conspicuously absent from the early textual record, appearing mainly in twentieth-century tellings.
Assessment: The ritual record and the monster record have different birthdays. That gap is the whole story.
Stories
The old man who knew
The tale: every year Nian devours villagers, until an old man reveals its fears: red, flame, and loud noise. The village survives, and the defenses become the festival's uniform.
Assessment: A tidy charter story that retrofits an explanation onto customs that never needed one.
Interesting claims
A monster manufactured to explain a party
The claim: Nian was assembled recently, perhaps early twentieth century, to give children a reason for the firecrackers, an origin story manufactured after the fact and adopted faster than the ancient customs it explains.
Assessment: The absence of early attestation is documented. Manufactured folklore is a recognized process, and this may be its most successful product.
Sources and further reading
Early New Year custom records: red couplets and firecrackers
20th-century popularizations of the Nian beast tale
Eberhard on Chinese festival folklore
Chinese folklore scholarship on the beast's missing early attestation
Worth asking
If the reason for your oldest traditions was invented a century ago, do the traditions change?