There are many Sinnoh myths in the Sinnoh region, below are several from Pokémon Diamond and Pearl:

Sinnoh Myth

Betray not your anger, lest ??? will come. Weep not with sorrow, or ??? will draw near. When joy and enjoyment come natural as the very air, that is happiness. Let such be blessed by the hand of Master ???. Those words were spoken often as customary.

It is generally suspected the ??? refers to Shaymin or Arceus.

Sinnoh Region’s Mythology

Long ago, when Sinnoh had just been made, Pokémon and humans led separate lives. That is not to say they did not help each other. No, indeed they did. They supplied each other with goods, and supported each other. A Pokémon proposed to the others to always be ready to help humans. It asked that Pokémon be ready to appear before humans always. Thus, to this day, Pokémon appear to us if we venture into tall grass.

Sinnoh’s Myth

Three Pokémon there were. Into the lakes they dove. Deep, deep, drawing no breath. Deeper, deeper they dove. Into suffocating depths they dove. Deeper, then deepest they alight. From the lake floor they rise. Bearing with them the power to make vast lands, they rise again.

Veilstone’s Myth”

A young man, callow and foolish in innocence, came to own a sword. With it, he smote Pokémon, which gave sustenance, with carefree abandon. Those not taken as food, he discarded, with no afterthought. The following year, no Pokémon appeared. Larders grew bare. The young man, seeking the missing Pokémon, journeyed afar. Long did he search. And far and wide, too, until one he did find. Asked he, “Why do you hide?” To which the Pokémon replied... “If you bear your sword to bring harm upon us, with claws and fangs, we will exact a toll. “From your kind we will take our toll, for it must be done. “Done it must be to guard ourselves and for it, I apologize.” To the skies, the young man shouted his dismay. “In having found the sword, I have lost so much. “Gorged with power, I grew blind to Pokémon being alive. “I will never fall savage again. This sword I denounce and forsake. “I plead for forgiveness, for I was but a fool.” So saying, the young man hurled the sword to the ground, snapping it. Seeing this, the Pokémon disappeared to a place beyond seeing...

Creation Story

In the beginning, there was only a churning turmoil of chaos. At the heart of chaos, where all things became one, appeared an Egg. Having tumbled from the vortex, the Egg gave rise to the Original One. From itself, two beings the Original One did make. Time started to spin. Space began to expand. From itself again, three living things the Original One did make. The two beings wished, and from them, matter came to be. The three living things wished, and from them, spirit came to be. The world created, the Original One took to unyielding sleep...

Myth

Look not into the Pokémon’s eyes. In but an instant, you’ll have no recollection of who you are. Return home, but how? When there is nothing to remember? Dare not touch the Pokémon’s body. In but three short days, all emotions will drain away. Above all, above all, harm not the Pokémon. In a scant five days, the offender will grow immobile in entirety.

Sinnoh Folk Tales

“Sinnoh Folk Story 1” Pick clean the bones of Pokémon caught in the sea or stream. Thank them for the meals they provide, and pick their bones clean. When the bones are as clean as can be, set them free in the water from which they came. The Pokémon will return, fully fleshed, and it begins anew.

“Sinnoh Folk Story 2” There lived a Pokémon in a forest. In the forest, the Pokémon shed its hide to sleep as a human. Awakened, the human dons the Pokémon hide to roam villages.

“Sinnoh Folk Story 3” There once were Pokémon that became very close to humans. There once were humans and Pokémon that ate together at the same table. It was a time when there existed no differences to distinguish the two.

Note

In the literal translation of Sinnoh Folk Story 3, the myth originally refered to Pokémon and people marrying each other.