The Mushrooms are a non-member den item. They were first released on April 2, 2015, and can be won as a prize in the Spring Festival.
The Mushrooms consist of a large blue mushroom with a smaller purple mushroom behind it and off to one side. The blue mushroom has light purple specks on it, while the purple one has blue specks on it. They only come in one variety.
There are three additional color varieties that are part of the game files, but they have never been released and they cannot be obtained by any legitimate means.