https://www.gamespot.com/articles/pokemon-manga-reveals-some-weird-unused-gen-i-mons/1100-6459053/

A Japanese manga detailing the origins of Pokemon has revealed some previously unseen monsters from the first generation, Red and Blue. The book, centered on the life of Pokemon creator Satoshi Tajiri, showed off four unused designs.

The manga was shared on Twitter by Game Freak's James Turner, who has designed plenty of Pokemon himself. In a grid of Pokemon we can see four unfamiliar designs: a deer, a crocodile, a cactus, and a swordfish or shark. Presumably, to get access to designs like this the manga would have had to collaborate with Game Freak itself.