The Agario skin appeared on July 27, 2016. Miniclip said on its Facebook page that the skin is for their birthday special. Agar.io was created on April 28, but Zeach sold Agar.io to Miniclip on July 27. It can be used by typing "Agario" in the Nickname Box. Your cell will then turn into a cupcake.
This skin changed on August 19th, when instead of cupcake you get a candle.