I know ARceus born before the spase exit, but how can he born?
No one knows. There are only theories with no evidence.
>It is said to have emerged from an egg in a place where there was nothing, then shaped the world.
>According to the legends of Sinnoh, this Pokémon emerged from an egg and shaped all there is in this world.
That's all we know. Arceus came from an egg that existed before the universe did, then proceeded to create it. Other than that it's theories and speculation. Did some higher power create Arceus? Did it create itself? Is he left over from a previous universe, a la Galactus? Was it just always there? We really don't know, and will most likely never know.
In monotheistic religions (Christianity, etc.), God was simply always there and came from nothing. It is highly likely that this is the same for Arceus because he is the "god" of the Pokéverse.
But like I said, we really don't know.
How come Arceus doesn't have the Pokerus Smilie Icon after he was cured?
New event moves for arceus?
What is the most useful and well rounded Nature for a single Arceus?
Was there an error in the anime with Arceus?
Would something bad happen if rock Arceus was un-banned in Monotype?