On the Synthetic-Reality forums, there have been Avatar Fads. Some fads are not listed.
Someone had a penguin avatar. Copies and variations soon followed. The penguins originally held a frying pan, but they were modded to incorporate a number of different items.
Someone made an avatar with a card. Variations followed, using mostly animae characters.
Someone had a carebear for an avatar, and I believe a temperary cult started.
The Sphinx avatar is put on people who abuse their Avatar priveleges. Someone more human than most new members got "Sphinxed," and from then, many members pretended to be sphinxed by manually changing their avatar to the sphinx.