From the Adventure Time wiki:

“Revealed on Natasha Allegri’s formspring account, the dress she designed for Fionna is based off Princess Serenity’s dress from the anime, Sailor Moon (as shown to the right). This may also be the inspiration for Fionna’s rabbit-themed hood, Sailor Moon’s protagonist is named “Usagi,” which is Japanese for “rabbit.” Usagi, or Sailor Moon, also has a talking feline companion named Luna.”