This sound comes from Fluttershy, a character from My Little Pony: Friendship Is Magic, the animated television series that became a major source for reaction clips and fandom memes in the 2010s. The whispered 'yay' spread because it perfectly matches Fluttershy’s timid personality and is instantly recognizable to fans. Online, creators still use it when they want a low-volume, adorable reaction instead of a big cheer, especially in ironic edits where a tiny accomplishment gets the smallest possible celebration.