The dress Fionna wears to Gumball's party bears a very striking resemblance to Princess Serenity's dress in the Sailor Moon anime. It should also be noted that Fionna wears rabbit ears and has a feline companion.... In Sailor Moon, Princess Serenity is reincarnated as Usagi (Japanese for Rabbit) a girl with a talking feline named Luna.
First appearance of Fiona and Cake, the "gender bent" fan fiction created by Ice King. All the characters are the opposite of their usual genders, so Finn becomes Fiona, Princess Bubblegum become Prince Gumball and so on.
The dress that inspired both Princess Serenity and Fionna's outfits is the Christian Dior Palladio dress from 1992.