YouTube user happysquidNET posted this video last November, which has been undiscovered until Redditor superangelo64 found it only hours ago. The track isn’t used on any playable track or menu screen. This has lead some people to theorize that the track may have originally been intended as an alternate credits sequence BGM.
This isn’t the first time Nintendo games have revealed interesting hidden content. For example, a while back Splatoon data miners found playable Octolings, as well as data for the Callie and Marie amiibo almost four months before their official announcement.
Regardless of what this piece of hidden Mario Kart 8 OST may have been intended for, it is now available for public listening thanks to happysquidNET. Keep up the great work, data miners!