Dragons at the Creation of the Great Hakurei Barrier
The last time dragons appeared in Gensokyo was when the Great Hakurei Barrier was created.
The sky was rent with constant thunder, the land was engulfed in a flood that threatened to submerge it, and for an instant, the skies were completely dark.
The youkai sages staked their existence on pledging eternal peace to the dragons, and in an instant the waters drew back, the sky was torn asunder, and light was returned to the heavens.
Since then, there has been a statue enshrining the dragons at the center of the human village, which is worshiped every day.