With the release of Pokemon Scarlet and Violet, the franchise has crossed the thousand Pokemon benchmark fans have been waiting for some years. All of this, of course, is built on the intense Pokemon lore established in the first few games, where time, space, land, water, and creation are all explained to some degree. The Pokemon universe is ever-growing, of course, and this makes the distinction of the first Pokemon more interesting. However, the answer to what is the first Pokemon is not so simple.
This is because, despite Pokemon regions having indicators about their timelines (such as the war in Kalos or the Darkest Day in Galar), most of the Pokemon timeline isn't concrete. For example, it's known that Pokemon Legends: Arceus takes place in Hisui, a name that existed before Sinnoh proper, but there's no real indication of how far in the past that is. At the same time, Pokemon Scarlet's Paradox Pokemon are all supposedly from the past, though some rumors suggest that the time travel machine is more of an "imagination" machine than genuine time travel. That aside, there are modifiers that are then dependent on how one looks at the question, meaning there isn't a simple clear-cut first Pokemon but a few contenders.
Bulbasaur is a Good, Simple Answer for the First Pokemon
Sometimes the simplest answer is the best one, and based on that conceit, Bulbasaur would be the first Pokemon. Simply put, it is the first Pokemon in the Kanto region Pokedex from the very first games. It's hard to argue that as answer, though some believe that Victini steals this claim from Bulbasaur. Ultimately, that argument doesn't hold water. Some will argue this because, in the Unova Pokedex, Victini has the Pokedex designation of #000. Zero obviously becomes one, but Victini's designation comes with caveats that Bulbasaur doesn't have. Obviously, Bulbasaur still appears in the first region, while Victini doesn't.
However, Victini's Pokedex designation is special in regards that it gained this number because it is considered lucky for its partner. It's not that there's some special lore reason it takes that spot; it's just a special designation by the professor of the region. At the same time, Bulbasaur still holds the #1 designation in the national Pokedex, whereas Victini gets the number #494. Whether one counts regional Pokedexes or just the national Pokedex, Bulbasaur beats out Victini every time.
Arceus is the First Pokemon for Obvious Reasons
While Bulbasaur is the simple answer when looking at it from a chronological order of Pokemon games point of view, another viable standpoint is the first Pokemon to exist in the lore. That is Arceus, the creator of the Pokemon universe and the being often considered to be the God of Pokemon. It created the legendaries that, in some way, shaped the universe. Dialga may be responsible for time, Palkia may be responsible for space, and Giratina may be responsible for antimatter, but Arceus is responsible for Dialga, Palkia, and Giratina. Arceus is just as viable an answer as Bulbasaur.
Why Mew is Considered the First Pokemon and The Flaws in That Argument
Some fans consider Mew to be the first Pokemon, but upon close inspection, that argument has some holes in it. It's not as ironclad as Bulbasaur or Arceus. On the one hand, Mew is the first Mythical Pokemon introduced to the series and that's valid. At the same time, many argue that all Pokemon start off as a Mew, which led to ditto, which led to various evolutions. This explanation isn't necessarily supported by lore though. Many believe this because Mew's Pokedex indicates it contains the DNA of every Pokemon ever discovered, thinking this means it is a progenitor in a way.
But that's not exactly true. For example, Mew can transform into Porygon and Mewtwo, among others, which are manmade Pokemon that didn't exist when Mew was first created by Arceus. It's always been a funny grey area in the lore, but also biologically speaking, Mew having all the DNA doesn't make sense in the way that Pokemon that followed it would contain some traces of its DNA, if more complex due to various evolutionary factors. This argument essentially is that because a child has dimples, the parent has dimples when the inverse is known to be true. For what it's worth, naming Mew as the first Pokemon because of this bit of funnily-worded lore and just fun fandom is perfectly fine; it just doesn't hold up to scrutiny like some others.
Rhydon is the First Pokemon in a Way
Interestingly enough, Rhydon has a similar claim to the first Pokemon title as Arceus and Bulbasaur. Bulbasaur is the first when considering the Pokedex, Arceus is the first Pokemon when considering lore, and Rhydon is the first when considering the real-world creation of the games. Rhydon was the first designed Pokemon, though of course, it underwent a couple of iterations. In fact, its original design would make it into the games as the Poke doll that appears when using the Substitute move. What's clear, no matter what, is the answers depends on the point of view, but there are definitely some that can be named as the first.