In Curse Of Strahd, Beucephalus appeared to Pack alongside his master, who used him to travel great distances quicker than any mortal Barovian could. Atop his back, Strahd used a Portal to take him to Vallaki and used him kidnapped Ireena Kolyana, before riding off on his flaming horse. Strahd again used Beucaphalus to quickly travel from Yester Hill to speak with Atlas Atruvian after The Pack destroyed Wintersplinter.
A mythical creature forgotten by most Barovians, Beucephalus The Wonder Horse was the childhood Warhorse of Strahd von Zarovich. Despite surviving countless battles with his rider, the horse eventually passed of old age, and was buried in the crypts of the unfinished Castle Ravenloft at Strahd's request.
After the Wedding That Never Was, the now-Vampire Lord began developing dark powers, and his mind twisted. Thirty years on from his horse's death, he engaged with the Lower Realms to transform Beucephalus's decomposed skeleton into a Nightmare. Ever since, Strahd could occasionally be seen riding across the Barovian night on Beucaphalus, and used his mount's ability to slip into the Ethereal Border plane to appear omnipresent.
Though Strahd kept the crypt in his catacombs dedicated to his "Wonder Horse", he generally instructed Beucephalus to remain close to him in the Ethereal, acting as a guard or mount at any given time.