Nero was a human man (or, according to certain accounts, an anthropomorphic dog) who is now a ghost.
Nero was one of the most dictatorial, crazed Emperors of the Roman Empire, having reigned in the 1st century. Through time-travel, he was encountered in the flesh by Mickey Mouse and by Gyro Gearloose at various points in his life. The organizer of extravagant public entertainment (such as flooding the Circus Maximus to organize crocodile fights), Nero remained most infamous for having played music as Rome burned around him, likely at his own initiative; this was recorded in the statue which accompanies his tomb in the Burial Crypt of Famous Villains in the Haunted Mansion.
Behind the scenes
Nero's first appearance in a piece of canonical Prime Universe media appears to have been in the 1962 installment of Mickey Through The Centuries, Hold Your Horses, Mickus!. Both this story, his planned appearance in the Tokyo Disneyland Haunted Mansion and his appearance in one Brazilian 1971 Gyro Gearloose story depict him as a human being, whereas in 1980 and 1981, Irineu Soares Rodrigues twice drew Nero as a dognose. Don Rosa additionally referenced Nero, without drawing him one way or the other, in The Crocodile Collector.