Reason: Skeleton dance part.
Walt Disney Animation Studios (also known simply Disney) is an American animated YouTuber best known for uploading Walt Disney Animation videos that became one of the most popular YouTube animator makers of the field.
The most viewed YouTube video to date is "The Skeleton Dance", a 1929 black and white adult animated horror cartoon by Ub Iwerks, which was (for a while), but not always, uploaded in October 2015, but it has been overthrown from that spot by others since.
Walt Disney Animation Studios' video The Skeleton Dance is the only video to not be for children and gave it a rating certificate of "X", prohibiting anyone younger than 18 from seeing the film. The rating has since been amended to "U" (Universal), stating the film as fit for audiences of all ages.
This 1929 cartoon video may feature violence, blood, scary things, and jumpscaring.