It was the early days of the universe. Arceus sat on his newest creation- Earth. He looked upon this boring planet, and realized that it was completely lifeless. He thought of life, and a single spirit came from inside him, and life slowly was created. He brought life to multiple pokemon, making mice to run in the fields, and birds to rule the sky.
Giant serpents and squids took over the sea, and pokemon made entirely of rocks walked through the caves.
Arceus sat, admiring the beautiful creations, when he noticed the spirit he had originally created. It was small, pink, and it was the thing that created life.
Arceus willed this pokemon a shape, and it slowly turned into a cat-like floating pokemon. Arceus told this pokemon "You shall be known as Mew. You will be the cause of all pokemon life, as you created them. You shall be the first of many powerful pokemon to rule over these creatures."
And so, Mew was created.