Arceus was created by ??? and then formed the universe as we know by giving birth to Dialga, Palkia and Giratina.
This process created time, space and a third component, which never came into use because Giratina misbehaved and was banished.
Arceus then created Groudon, Kyogre and Rayquaza, to form the earth, sea and sky.
To inhibit this new planet, Arceus created Mew together with Azelf, Mesprit and Uxie, which explains why they are all psychic type.
Arceus also created the concept of life and death with the birth of Xerneas and Yveltal.
Mew started to reproduce by using its own DNA, over time more and more different species of Pokemon evolved from this.
While Groudon and Kyogre were fighting, Mew was hurt and some of its cells scattered, but it regenerated and survived. This fierce fight, and Rayquazas involvement, resulted in Mew cells being hauled into space. This resulted in alien Pokemon like Clefairy and Elgyem, which of course are pink and/or psychic type, like Mew.
One piece of Mew DNA, however, was sealed inside a triangular stone, which Rayquaza created by accident when reverting back from its Mega form.
Millions of years later, in a great war, the power of Xerneas was used through a weapon, which hit the triangular sealed stone. This resulted in the Mew cells becoming a new Pokemon named Deoxys, which of course is also psychic type.
All other legendaries dont really serve a universal, recreational or administrative purpose, so they were not created by Arceus.
Life just took its course.
Or we just say everything after gen4 is not canon anyway because it sucked.
