Quote : " If you're running the Windows 10 Mobile Insider preview on a Lumia 930, 1520, or Icon, you can now try slow-motion video with the updated Windows Camera app. "
Source : http://www.windowscentral.com/windows-camera-slow-motion-video-hands-on
If I understand this correct it's the builtin Camera app itself and not any extra. Also very interesting that this is not included with the very latest Windows Lumia versions 950 and 950 XL.