I'm not really sure if there are specifically 'popular' 'pokemon slave fanfic' stories. Given the rather dark and potentially controversial nature of such a concept, it might not gain mainstream popularity. But in the niche world of fanfiction, there could be some that have a following among a small group who are interested in exploring these darker themes.
Honestly, I haven't come across any highly popular 'pokemon slave fanfic' stories. The idea of pokemon in a slave - like situation is quite a departure from the norm. The official Pokemon world promotes a harmonious relationship between humans and pokemon. Fanfics that deviate too much from this positive image may not attract a large audience. However, in the underground or more niche areas of fanfiction, there could be some that exist, but they are not likely to be well - known to the general Pokemon fan base.
It's difficult to say. The Pokemon franchise is mostly known for its positive and family - friendly content. So, a 'pokemon slave fanfic' goes against that general image. While there may be some out there in the vast ocean of fanfiction, it's not likely to be widely popular or promoted. Most Pokemon fans are more interested in stories that follow the traditional themes of friendship, adventure, and the bond between trainers and pokemon.