Pokémon: Detective Pikachu brought numerous realistic versions of Pokémon from several generations to the big screen, delighting fans. Apparently, however, more creatures were planned for the film, including Golbat from the first 151 Pokémon and Gardevoir from the franchise's third generation.
In an interview with ComicBook.com, writers Dan Hernandez and Benji Samit gave more details, explaining that not every Pokémon they wanted to include made it into the final version of the film. This is understandable, considering there are more than 800 to choose from and the film does not run for two hours.
Hernandez mentioned that Golbat, the evolution of Zubat, would have appeared in a scene he particularly liked:
For a while, we had a cool scene with a Golbat that I really liked. It was a really cool scene that simply did not survive until the end of production.

A realistic version of Golbat by artist Joshua Dunlop
Samit also said in the interview that Gardevoir, a very popular Pokémon, was cut as well. Not every dream met that fate, however: he explained that the writers' priority, Psyduck, was approved by the rest of the team and ended up with a prominent place in the film:
Gardevoir had a cool moment. We had so many different drafts throughout the process... To be honest, we are happy that our favorite made it into the film, and that is Psyduck. At our first meeting about the project, it was not involved yet, and I think the first thing we said to the producers was, “Psyduck needs to be in this film, be Lucy's Pokémon partner, and be the comic relief character.”

Gardevoir in its appearance in Pokkén Tournament
None of the scenes were explained, but this may be because they could be included in future films, considering that a sequel to Detective Pikachu is already being written. Fans certainly hope these and many other Pokémon will still make their debuts in the film's peculiar and colorful universe, and only time will tell which ones get that chance.