The Agario skin is a skin that appeared on July 27th, 2016. Agar.io said on its facebook page that the skin is for their birthday special. Agario was created on April 28th, but Zeach sold Agario to miniclip on July 27th. 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 cucpake you get a candle.