After the discoveries of other moons of the other planets, they gave them names. Each planet that has a series of moons i.e. Jupiter, Saturn, Mars and so on, have names that are in relation to their planet. God of war had Phobos as is Jupiter with Callisto and Io etc. Our moon is number 1 moon, hence the name moon.
Many cultures have names for the moon.
As Luna, and Pyra the brother to the Sun from certain NA Indians. Rana from India a goddess of some sort. I call it Fred the Moon myself. After the discoveries of other moons of the other planets, they gave them names. Each planet that has a series of moons i.e. Jupiter, Saturn, Mars and so on, have names that are in relation to their planet.