Any idea what this is and what it looks like/how i do open it?
yes, those are Piñata's, they break on any damage, so your AOE's are probably popping them.
They are like the 'super chests' you sometimes find in dungeons.... Except they pop on damage so they are easy to miss.
There is usually one per whimsyshire - and I have either missed it, on a couple of my run throughs, or it didn't spawn, I have never seen two in one whimsyshire.
Threats of violence. We take these seriously and will alert the proper authorities.
Posts containing personal information about other players. This includes physical addresses, e-mail addresses, phone numbers, and inappropriate photos and/or videos.
Harassing or discriminatory language. This will not be tolerated.