This is Nimbles pony disguise! He uses it to race the pegasus and blend in with ponies in general! Discord and Dozy and by extension their children were all urged by Princess Twilight to take on Pony disguises to help reduce the chaos that a draconequus brings into pony settings and to help reduce the prejudice that they faced (a bad move imo). So each of them have created a spell that disguises them as a pony race of their choosing. Some of them (like discord) can maintain the disguise as long as they want, for hours or even days, and some like Dozy, only a few hours at the longest, before their magical energy is drained and they have to take a break or the spell will begin to falter. Nimble can hold the spell for about 12 hours, a bit longer if he pushes. He mostly uses the time to race, as he is a very fast flier in and out of his pony disguise. He forms friendships and rivalries with ponies while in his disguise and eventually meets and falls in love with a mare named Harper who he races against.