Confession: I had schemed to have Ellen want to have a tail, because tails are nifty, and I thought Ellen occasionally having a tail would also be nifty. While that is true, having it linked to April Fools nonsense would have been sort of horrible.
It's not the first time I've contemplated giving a main character a tail, however. In the prototype EGS comics I made back in high school, the "incident" between Tedd and Sarah resulted in Sarah being temporarily transformed into a half-Pikachu (no, really), and when she returned to mostly normal, she inexplicably had a prehensile tail. Not wanting to risk winding up with something less desirable, she decided to just stick with having the tail, which didn't seem to bother anyone.
Could something like that wind up happening? Not in the same way, but I wouldn't put it past me to have someone somehow wind up with a tail as part of their default form.
Commentary added Dec 30, 2014