About legends

In the beginning, it was only pokemon.

Arceus created the original ancestor mew, along with the other legends to keep order over their various domains. Humans did not exist, and for the most part, pokemon were at animal level intelligence, including many of the legends - though this didn't cause many problems, because pokemon were the only creatures who existed.

But when Arceus met with the Sky Mother, she gave their world humans as her gift. At the time, the humans were relatively disorganized and on about the same level as the pokemon. But when the humans banded together with themselves as well as pokemon, they were able to create large communities where they could stay instead of constantly moving. Humans had the skill to coordinate teams, so pokemon would join and begin living with humans as companions and helpers; because the humans had the knowledge to help them improve.

Because of this, an issue soon arose: while most pokemon were willing to be friendly with humans, many legends were not - and to have an all powerful divine being be on bad terms with a population is, well... not good. Legends and humans (along with teamed up pokemon) would often engage with eachother, and quite often for the worse. In addition, some groups of humans had actually found out how to kill legends.

At the point in which this happened, Arceus has already severed their emotions, but they knew balance couldn't be kept in the state things were, and decided to unify legends as beings of both human and pokemon.