When the earth was fully formed and life took its stage, the Rattengott came into existence. A speck of dark emerged from the black hole and dropped into the newly made earth. Upon arriving on earth, the speck landed in a volcano. The volcano erupted and a giant creature of darkness climbed out of it. His name is Rattengott.
During his stay on earth, he decided to create an animal species based on his appearance after seeing how many animals were created. This is how the rat species are made. Then he hid in the shadows and watched them over the years as they grew and birthed many new rats.
During the early middle ages, he saw humans for the first time.
While watching over the rats and protecting them from creatures that hunt them, He heard some beautiful songs from the town. Rattengott had never heard music before but he found the sounds wonderful. He follows the music and finds a band of travelling musicians playing to the children(He hid in the shadows so they wouldn't notice him). Rattengott was fascinated by the musicians and wanted to know how they created music he had never heard before.
But Rattengott knew his appearance would scare them off. Other animals ran away when they saw him for the first time. So he decided to transform to fit in with the humans. Using his shapeshifting powers, He changed into a human man and the rats gave him the clothes he needed after they stole them from the clothesline.
He walked over to the musicians and introduced himself as a lonely traveller with no name. He then joined the Musicians, who taught him how to use music for the first time. Once he mastered all the instruments and had full knowledge of music, he then officially became a new member of the musicians and was given a new name.
He continues travelling with the musicians from town to town, playing music and entertaining the audience until the band slowly breaks up. Some died, some settled down with new families and some left to become their own musicians. Rattengott wasn't sure what to do now the band broke up and went their own paths but he then decided to go back to his true role as a god watching over the rats.
But Rattengott later began missing his years of being a human so he turned into his human form again and it has become his default form ever since.