Monsters is a oneshot manga by Eiichiro Oda written in 1994. It is canonically part of One Piece, although this was not revealed until One Piece’s Thriller Bark Arc and confirmed in SBS Volume 47. 
Even though the story was first written and published in 1994, it was released in November 4, 1998 as part of the Wanted compilation of oneshots. Its initial release date in 1994 is unknown.
* The swordsman “Ryuma” who was once said to have defeated a dragon in One Piece is the very same Ryuma that starred in this short story “Monsters”. And he looks exactly like Zoro.