The Herbivorous Old Dragon is Named the Evil Dragon - Coming Out of the Mountains and Entering the World

Shicao Lao Long Bei Guanyi E Long Zhi Ming - Chushan Rushi Pian
食草老龙被冠以恶龙之名-出山入世篇

Source

A Gentle Dragon of 5000 Years Old, It Was Recognized as an Evil Dragon without Any Cause.
Yowai 5000-nen no Soushoku Dragon, Iwarenaki Jaryuu Nintei: Yada Kono Ikenie, Hito no Hanashi o Kiite Kurenai· 齢5000年の草食ドラゴン、いわれなき邪竜認定 ~やだこの生贄、人の話を聞いてくれない~
novel

A 5000-year old vegan dragon was living peacefully when one day a young girl showed up in his cave. She offered herself as a sacrifice in order to gain favor for her village. He played along as the great "Evil Dragon, the Demon Lord's Army Leader" in order to get rid of her. However, his little white lie awoken her hidden powers and his peaceful life suddenly ended.

Alternative

A Herbivorous Dragon of 5,000 Years Gets Unfairly Villainized
Yowai 5000-nen no Soushoku Dragon, Iwarenaki Jaryuu Nintei: Yada Kono Ikenie, Hito no Hanashi o Kiite Kurenai· 齢5000年の草食ドラゴン、いわれなき邪竜認定 ~やだこの生贄、人の話を聞いてくれない~
manga

For 5,000 years, a harmless herbivorous dragon lived a quiet, secluded life. But everything changed when a girl called Reiko appeared before him. She offers herself as a sacrifice to who she believes is the evil dragon, Revandia, in exchange for helping her defend her village from the demon king. Together, the so-called “evil” dragon, devoid of flight or magical prowess, and the enigmatic girl embark on an epic, demon-vanquishing adventure! *Source: Manga UP! Global*