Per copyright law in the United States, the designs must be 15% different to create one's own artistic copyright. They're several that are close, but 15% isn't that much. Obviously Nintendo isn't a US company, but it'll be interesting to set how this goes
Both Nintendo and Pocketpair are Japanese companies and their copyright laws are MUCH more harsh. In fact Japan even lacks concepts of "Fair use"
Add to that the fact that public opinion also matters intensely in Japanese courts (think juries) and that Pokémon is considered a national treasure and well... Things are stacked against Pocketpair