The most likely explanation is a coordinated ARG (Alternate Reality Game). A group of Galician artists or gamers created "FU10" as an immersive horror project. The name "FU10" is suspiciously convenient—it sounds like "eff you ten," which could be a trollish inside joke.
The persistence of the myth rests on three verifiable misunderstandings : fu10 the galician night crawling verified
Regardless of its reality, the keyword has exploded. As of this writing, "fu10 the galician night crawling verified" sees over 5,000 monthly searches, peaking during the Galician winter (November–February), when nights are longest. The most likely explanation is a coordinated ARG