The official announcement reads like this:
Walt Disney Animation Studios presents a never-before-seen short starring the one and only Mickey Mouse in Get A Horse! featuring Walt Disney himself as the voice of Mickey Mouse, this black-and- white, hand-drawn short follows Mickey, his favorite gal pal Minnie Mouse and their friends Horace Horsecollar and Clarabelle Cow as they delight in a musical wagon ride, until Peg-Leg Pete shows up and tries to run them off the road.
The implication is that this is a recently discovered classic. Look at the title card.
When the film premieres at the Annecy animation fest in June, amongst the guests will be Eric Goldberg, who is one of the very few remaining hand drawn animators still employed at Disney; producer Dorothy McKim; and Lauren MacMullan, sometime director on The Simpsons.
Note: edits have been made to this story since publication.