Pokémon: Detective Pikachu featured appearances from the Kanto starter Pokémon and their fully-evolved forms, but some of them had more prominent roles than others. Charizard had a major scene in the movie where it fights Detective Pikachu in an underground Pokémon Fight Club, yet Venusaur was relegated to a background cameo.
Venusaur was shown in the "Casting Detective Pikachu" trailer, which revealed the new designs of many of the Pokémon that appeared in the film. The new design for Venusaur was about what you might expect for the realistic style used by the movie, but the producers had another design that they could have used (thanks, Kotaku).
One of the designers on Detective Pikachu was an artist named RJ Palmer, who is known for his realistic Pokémon drawings that were shared by many different websites. Palmer recently uploaded a picture to Twitter that showed one of his original designs for Venusaur that went unused.
This was my design for Venusaur on Detective Pikachu. This was during a period where I was free to explore a more naturalistic approach. Looking back though its super clear that this would be too scary for the tone of the movie. I think the design they ended up with is fabulous! pic.twitter.com/SSekVKI47D— RJ Palmer 🔜 Anime Expo H3 (@arvalis) May 28, 2019
Palmer claims that the original design for Venusaur may have been too scary for the movie, which means that he clearly never saw the terrifying Lickitung that appeared in the film, as that was way scarier than the big dinosaur/frog that he drew.
The revamped Venusaur design that appeared in the movie had a smaller head, larger eyes, a smaller mouth with fewer teeth, and fewer leaves on the plant on its back. The big mouth & small eyes of the original design is likely what prompted the redesign, as it's the opposite of the aesthetic favored in Japanese animation. The Bulbasaur that appear in the movie have large eyes that are intended to make them seem cute and the original Venusaur design would have made the creatures far less appealing.
The majority of the Pokémon that appeared in Detective Pikachu were small or man-sized creatures and there weren't many huge Pokémon shown, save for the mutated Torterra scene. The future live-action Pokémon movies have been left with a lot of Legendary Pokémon that have yet to be shown on film, which have the capacity to be the big and scary Pokémon that Venusaur could have been.
Pokémon: Detective Pikachu is in theaters now.