You’re thinking about a certain Disney dog, aren’t you? Goofy’s pet dog? Nope, it was actually named after Pluto, the Roman god of the underworld.
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When Pluto was first discovered by Clyde Tombaugh in 1930, he was given the honor of giving it a name. Although they were calling it Planet X informally, they needed something that matched the rest of the planets in the Solar System.
The name Pluto was suggested by Venetia Burney, an 11-year old school girl in England. She was interested in ancient mythology, and thought that Hades, the Greek god of the underworld, made a good name. She suggested Pluto, to match the Roman god names given to the other planets.
Each astronomer in the Lowell Observatory was allowed to vote on a short list of names: Minerva, Cronus, and Pluto. Every one of them voted for Pluto. Venetia was given a 5-pound reward for providing the name.
In other languages, the name has been translated to names that match underworld god mythology, such as Yama, the Guardian of Hell in Buddhist mythology.