The Japanese shonen series One Piece is one of the longest-running productions of all time, and its 900+ episode anime adaptation would take over 15 days to finish according to CBR. It has also teamed up with the ever-popular Pokemon franchise before, with Pokemon GO adding One Piece-themed content to raise money for Kumatoto earthquake relief in 2019.
One Piece has retuned the favor by apparently having the Mythical Pokemon Meltan appear in its ongoing manga. The Steel-type "Hex Nut" Pokemon can be found clinging onto the shoulder of Captain Kid in Chapter 981 of the manga, and then it flies off as the character turns to direct an attack using a large mechanical device.
Apparently this is an ongoing easter egg, as sites like ComicBook reported that two Meltan appeared within more busy, mechanical scenes in Chapter 980 as well. Meltan and its evolution Melmetal are exclusive to Pokemon GO, but they can be transferred to Let's Go, Pikachu and Let's Go, Eevee on the Nintendo Switch before being added into Sword and Shield through the cloud storage service Pokemon Home.
Eiichiro Oda, the creator of One Piece, wrote in an author's comment back in 2018 that he has been playing Pokemon GO from his house since it first released according to an archived image by MeanMustacheMan on the r/OnePiece subreddit. Knowing this it makes perfect sense why One Piece crossed over with Niantic's AR mobile game, and why Pokemon is showing up in One Piece even with Pokemon GO now focused on introducing Galarian Forms.