Dance Dance Darkmoon
From Dancing Machine. I have made a weak aura helper that makes it easier to do this, especialy if you are planing to fast spam it just for achievement.
The 
Dance Dance Darkmoon is a toy rewarded from the Darkmoon Faire achievement Dancing Machine, introduced in patch 9.2, and was later hotfixed to be classified as a toy shortly after launch.
Guide awaiting review
Earn Dancing Machine Achievement
During the Darkmoon Faire, participate in the Dance Dance Darkmoon activity. While your opponent performs their dance moves, click the icons that appear in the center of the screen to input the sequence, then spam the center button when it is your turn to perform.
Collect Your Mailed Reward
Once the Dancing Machine achievement is completed, the toy will be mailed to your in-game mailbox — check your mail to retrieve it even if your inventory was full at the time it was sent.
Tips
See the video guide for a visual walkthrough of the dance input sequence.
