Board Thread:Questions and Answers/@comment-27625479-20161007114212/@comment-34835078-20180227043927

The Elf Tail Armor was released in March 2012. Some colors were re-released in 2015. The colors which were not released became known as "Beta Tails."

This is the source of the information:

http://aj-item-worth.wikia.com/wiki/Elf_Tail_Armor