Apparently the word "Derpy" has been banned in Xbox Live usernames according to someone attempting to change their tag to "Derpy Hooves". Apparently having "Derpy" in the name made it illegal.
Support noted that the users current name with Derpy in it would be grandfathered, but if he changed it he wouldn't be able to get it back.
Why they banned Derpy will probably forever be a mystery, but for now, you will have to pick a different name.
Thanks to The Bat Pony for the heads up.
