caramelldansen
ウマウマ ウッーウッーウマウマ(゚∀゚) キャラメルダンセン The_Caramel_Dance
A Swedish dance/techno song by Caramell. The song's title translates to The Caramel Dance.
The song spread quickly on the Internet, after a sped-up version by DJ Speedycake was paired with a dancing Mai and Mii from Popotan in a Swedish-made fan-edited flash video (Danbooru post #113333), which was later reposted to Nico Nico Douga. The dance is called "ｳｯｰｳｯｰｳﾏｳﾏ(ﾟ∀ﾟ) (U-u-uma uma)" or "ウマウマダンス (Uma uma dance)" in Japan, spreading even further there following Exit Tunes obtaining the rights to the song from the original producers, releasing the original and sped-up remix to great success.
The vast majority of posts under this tag reference the visual part of the flash, which comes from the opening animation of the Popotan game and has nothing to do with the Caramell song. Interestingly however, later on, Caramell (or rather their producers) did put out follow-up music videos of the song featuring anime/manga inspired personas, known as the Caramella Girls, doing the dance, with the exact same tempo as the original edit.