This building was announced on the 8th of August in 2012. It is called a "Pure Terra Habitat". Most likely, it was a habitat that housed only dragons whose elements are solely Terra or the Pure Terra Dragon It had a higher max gold storage than any other Terra habitat, but strangely only provided room for 1 dragon.
The second building was announced on the 9th of August in 2012, and is called a "Pure Flame Habitat". This was most likely the same as the Pure Terra Habitat, except that it is for the Pure Flame Dragon.
The third building was announced on the 14th of August in 2012 and it is called a "Pure Sea Habitat". This was again most likely the same as the other Pure Habitats, except that it is for the Pure Sea Dragon.
The fourth building was announced on the 16th of August, 2012 and it is called a "Pure Nature Habitat". This was again most likely the same as the other Pure Habitats, except that it is for the Pure Nature Dragon.
These habitats were available for purchase in the shop for 1.5M gold until the Light and War upgrade. Also released were Pure Habitats for other Pure Elementals -- the Pure Electric Habitat, the Pure Ice Habitat, the Pure Metal Habitat and the Pure Dark Habitat.
After its release, Players realized that the reason for the 1-Dragon limit was the high rate of generation Pure Dragons possessed. The Habitats were replaced with the Pure Habitat during the LaW update, although the replacement was purely in specs. The Storage System still treats the Habitats as a Individual Habitats -- it will not group with other Pure Habitats that were originally for a different Element.
Awhile back, there was a rumour which involved the "upgrading" of the mystery buildings. Some believed that if there was an upgradable sign, more dragons could be placed/mixed in the mystery buildings. The mixing and placing of dragons is not available for action.