Isn't One-Eyed Jack the heroic identity of one of the Loud House kids in one episode where they all take on card-themed personas?
Here's a fun experiment to try. Go back to Excelsior and instead of trying to create a new character, just try to rename an existing character with the taken name. If the name is taken, it will tell you so, but it might give you a message telling you that it's not an appropriate name, which occurs if you try to use no-no words but also if you try to take something that's been trademarked and put on a list. Superman isn't technically inappropriate, but you can't have it. So it's possible that between the time you created these characters and when you tried to move them, the names were added to the list of blocked names.
If that's not the case and you get a message saying that someone already has that name, you can always log in and use the /getglobalname One-Eyed Jack command to find out who took it and ask "What's up bro?"