As to why it's Poison and not Ghost: Frostzi is based specifically on the zombies popularized by the George A Romero films (Night of the Living Dead, Dawn of the Dead) and Hollywood, which come about through viruses and not supernatural/paranormal means.
Dawn of the Dead also sparked a series of zombie films in Italy.
The Shutder line loosely inspired by a childhood memory of mine. I grew up being really into movies. My whole family was. My older brother in particular was really into horror films (Polterture's hunting behavior being inspired by found footage horror). So back in the 2000s, my brother had the book "1001 Movies You Must See Before You Die". I was around 10 or 11 years old at the time and I was skimming through it, reading the small summaries about the films and their history that the book included. This is how, at a very young age, I came to learn about Cannibal Holocaust and 120 Days of Sodom, two very infamous movies from Italy, the basis for my region. Luckily the book only dedicated about half a page to them, and lucky for 10 y.o. me I was actually able to register that these were too disturbing for me at the time.
Cut to the 2020s and a close friend of mine, a huge film buff, was telling me about Mondo films (Mondo is Italian for "world"), a subgenre of exploitative documentary films with Italian roots. This is where the Shutder's Pokedex entry stems from.
The shinies are based on sepia-toned film.