If your a new fan to Adventure Time you might not know about the fine Fionna or her feline companion Cake. They're girl versions of Finn and Jake created by Natasha Allegri a crew member on the show. She created them just for fun.
This might be there earliest appearance online.
Here's Natasha's site and pic of her below.
Cake and Fionna are in the gender bending universe of Adventure Time where everyone's gender has switched. So if your a boy like Finn you'll be a girl like Fionna in the other universe.Madeleine Martin as Fionna, Roz Ryan as Cake, Grey DeLisle as the Ice Queen and Neil Patrick Harris as Prince Gumball.
bit about the episode, too.
" I fought for Marshall Lee( male version of Marceline)- I wrote him lines but they were cut for time! I practically begged Pen to let me work overtime to put him back in the episode, even in some small way. I wanted him to be voiced by Dante Basco"
" Adam and I really worked to make this one special. We changed it a lot, the outline actually had no Ice Queen in it at all! No ball, no crystal sword… act two used to be a date in a restaurant! We both knew we wanted more fighting, and if there had to be a date, it would be a date with swords and skulls and fighting!"