While technically not a "form" in the strictest sense, "The Tale of Genji" (written by Murasaki Shikibu, a lady-in-waiting at the Imperial Court) established the foundation for a distinct genre of Japanese literature: "monogatari".
Monogatari is a genre of Japanese prose narratives, often focused on romance, courtly life, and societal dynamics. "The Tale of Genji" is considered the pinnacle of this genre, and its influence is evident in countless later works.
Therefore, while "The Tale of Genji" itself isn't a new literary form, its creation and popularity helped establish "monogatari" as a significant genre in Japanese literature.