While the only operating system Microsoft was talking about at Mobile World Congress 2015 was Windows 10, the only operating system on the many Windows Phones on display was Windows Phone 8.1.
The only exception was French OEM Alcatel, who actually showed off the recently announced Alcatel One Touch Pixi 3 running the preview version of Windows 10 Mobile.
The Pixi 3 is a device which is available in both Android, Firefox OS and Windows Phone and features LTE support, a 5-inch FWVGA display, quad-core processor, 1GB of RAM, 4GB of onboard storage expandable, and an 8-megapixel rear camera and 5 megapixel front facing camera.
While the device shown at MWC did not run a version different from the public Technical Preview, I note the presence of numerous hardware development apps in the start menu, suggesting the OEM was already working on integrating the new OS with the device. Hopefully this suggests they may be one of the first out of the gate when the OS is finalized.