The next-generation Xbox may be codenamed "Durango."
Crytek senior technical designer Sean Tracy wrote the following message on Twitter: "Enjoying the Durango developers summit in London. So far, great swag and interesting talks." Tracy deleted the tweet soon after it went live.
Keep in mind that Durango likely wouldn't be the final name of the project, similar to the Wii U's Project Cafe codename.
Previously, the successor to the Xbox 360 has been largely referred to as Xbox 720, though that codename has never officially been sanctioned by Microsoft. Rumors last year also suggested that the system was codenamed "Loop."
For now, Microsoft hasn't made any announcements about its next console, and all codenames are only a rumor.