Dancing Skeletons are easily differentiated from other Skeletons as they, well, dance. There are similarities in their dance moves to that featured in the Gangnam Style music video.
In order to join them in their jig, simply get hit by one and the achievement will unlock.
If you're struggling to find any, they can be found in any of these areas:
Cemetery Ground
Defiled Chapel
Forlorn Catacomb
The High Quarter
The Merchant Quarter
Weeping Halls
Widow's Mine