I'm not sure if this is the way it's supposed to be or not, but I want to create a pet as a Cadogre or Endeavor (so that it can be genderless) and the name I want belongs to an existing pet, but is on an inactive account (The message at the top of the pet's profile says "This pet belongs to an inactive account! You can create a new pet, or change a pet name to this name!")
However, when I try to name my new Cadogre or Endeavor this name, I get a 404 saying "Oh No! Someone already owns (Pet Name), sorry!"
This appears to be an issue with the special species that has been reported to the programmers.
You're welcome to add your report here: * Can&;t create a pet with a inactive pet&;s name
You might even consider filing a ticket to see if a UA has any additional options for you until it's fixed.