The tag says "(c)1998 Nintendo, CREATURE, GAME FREAK, TV Tokyo, ShoPro, JR Kikaku Licensed by Medialink International Limited." It's manufactured from "Lucky Well International Ltd," if that also helps.
It seems he's made of taffeta although there isn't a specific material named on the tush tag. I'm thinking that he may be a Mirage Plush, but I'm not entirely sure. He could also be a bootleg, perhaps, but I don't have the expertise on these things. I'd appreciate any pointers and help from the community! This has stumped me for years.