While the electronic model popularized by Professor Samuel Oak is what comes to mind, in truth, the idea of the Pokedex is much, much older. In fact, the oldest surviving "Pokedex" is one of the eleven volumes of Naturalis Historia, written by Pliny the Elder in 77 AD (estimated). Though his methods were... bizarre, and his research extremely incorrect, the value of his work can not be overstated.
Through varying eras and locations, building off of Pliny the Elder's work, the idea of a Pokemon encyclopedia was an appealing one throughout history. Many, many people tried to make a definitive encyclopedia, but Pokemon simply change too quickly, and can escape too easily, for proper study. The most complete known Pokedex from history was Professor Laventon's Encyclopedic Monster Index of Hisui (estimated circa 1850).
The modern Pokedex, however, came about in 1996 from Professor an idea of the aforementioned Professor Oak. Abandoning the traditional methods of individuals using careful information, Professor Oak's idea was groundbreaking- A series of sophisticated measurement tools in a small, handheld package, connected to a Trainer's Pokeball system, allowing a Trainer to input data with incredible ease. Though this system was not immune to misinformation or mistaken measurements, overall, it became much easier to gain data. Professor Oak then entrusted this original model Pokedex with a Trainer from his hometown named Red, who then became famous for his dismantling of Team Rocket.
(It is known that Professor Oak made two of these original prototype Pokedexes, the fate of the second is unknown- Perhaps he kept it, or gave it to a family member, or simply scrapped it.)
However, this original Pokedex model, known as the model HANDY505, was merely a prototype, and as a result riddled with problems. It only had the space for around 150 species of Pokemon in its memory, which was barely sufficient for only the Kanto region- and for that matter, it had no recording ability for locations or habitats outside of Kanto.
However, Red's collection of Pokemon and filling out of the prototype was more than sufficient proof of concept for Professor Oak to receive more League funding, and he set about improving the Pokedex to a vast degree- The National Pokedex (often argued to be called the International Pokedex) was completed, and the sensor and measurement tools of the Pokedex were refined, even being able to extrapolate probable habitats from captured members of a species.
With the completion of the model HANDY910 used in Sinnoh, however, Professor Oak seemed to have finally been content with the advancement of the Pokedex, and in 2007, he did the one thing that would change the study of Pokemon forever:
He made the Pokedex open-source.
Now many researchers and scientists worldwide can create their own Pokedex. And each version of the Pokedex connects and works with each other, allowing for linking and gathering data on Pokemon in ever greater quantities than before. Whereas in Laventon's research, he had to catch vast quantities of Pokemon himself and study each of them individually, the Open-Source Pokedex Project allowed for scientists and professors worldwide to receive massive amounts of data from individual species just by simply looking into the collected information from the massive amounts of Trainers in the world, crowdsourcing research.
To this day, many variants on the Pokedex are still being created, trying to achieve the most useful and most complete version. Alola has a Rotom inhabit its Pokedex, allowing greater recording of information while the Trainer's hands are freed to focus on the Pokemon itself. Galar managed to compress all of this into a phone application, losing some sophistication in measurement tools in favor of greater convenience. Paldea's takes the form of a "digital bookshelf", where each Trainer's notes are recorded into books and sent to a central location for study. And the rush does not seem to be dying down- Every region, every scientist, seems to want their personal version of the Pokedex to be the best, to finally be the most complete, to know all there is to know about Pokemon.
Studying and learning everything about Pokemon is how we humans express our love for them.
