To date, no conclusive evidence for a person or a group discovering all eighteen of Eevee's evolutions exists before the fourteenth century CE at the earliest. It is easy to see why.
One of the most obvious reasons is the Eevee evolution family's instinctual wariness of humanity for much of our species' history. This wariness ensures a self-reinforcing barrier to understanding Eevee; little is known about them, so it is hard to know how to go about obtaining more information.
The species' low birth rate relative to all other Pokemon, to say nothing of other Field Egg Group Pokemon, was another factor, as was the gender imbalance in the species. The sexual dimorphism of the species is not the most obvious to the unknowing, which was but the first hurdle in starting a successful breeding program that meant that, seven times out of eight, the attempt had to restart from the beginning, at least.
Furthermore, Eevee kinship groups – usually calling themselves families, but sometimes clans or houses – often increased their fitness for an environment and their claim to territory by seeking to only evolve into one or a handful of specific evolutions. Over enough time, this transformed into a social expectation among members of the group, evolved or not, that pressured individuals training to evolve to choose said specific evolutions or face ostracization. Additionally, the species' knowledge of its evolutions is less instinctual and more learned than species with far fewer evolution lines. Furthermore, rival kinship groups from nearby geographies picked different specific evolutions, leading to further social pressure to not be seen as an enemy or traitor. Subsequently, these kinship groups might only be intellectually aware that they can achieve evolutions other than the ones their societal norms say they can.
Thus, it was not uncommon for people, and the Pokemon themselves, to only believe Eevee to have a few evolutions, especially in rural or isolated geographies.
As always, tales and legends followed in the wake of Pokemon wherever humans saw them. Tales in the mountainous, northern parts of Johto and Kanto, where Water, Fire, and Thunder Stones are particularly abundant, often tied the three evolutions of Eevee they knew of to the three Legendary Beasts, Suicune, Raikou, and Entei. The latest incarnation of the story in Johto involves the burning of the Brass Tower and the Legendary Pokemon Ho-oh.
Another Japanese story explaining the evolution of an Eevee in a particular area is the one told by the Draconid tribes living in the Hoenn region. The gist is that a family of Eevee that acted much like the Legendary Pokemon that region's people most often revered, Rayquaza, Kyogre, and Groudon, evolved to become more like the Legendaries they idolized, with the younger twins Vaporeon and Terreon devastating their homeland while the elder brother only evolved into a Drakeon to stop their siblings.
The story differs in other areas of the Hoenn region that favored a different member of the weather trio. Where Groudon was revered, it was often a Terreon or a Flareon that triumphed over the other two members. The same held true with those who revered Kyogre and claimed Vaporeon triumphed, though the Draconids' versions of the story were the only ones that featured Drakeon instead of the more relatively common Jolteon or Zephyreon.
Regardless of any other reason, however, the main reason that no one discovered them all before the globalization and subsequently increased flow of information brought about by the birth of the First Mongol Empire was that there were very few places in the world that contained the correct conditions where an Eevee might achieve evolution into any of its eighteen evolutions within a relatively restricted geography… without first evolving into some other evolution.
Anyone who has tried to evolve their Eevee into Eeveeon via level up – without an Everstone – can attest to how painstaking it is to keep an Eevee from evolving into anything else before its normal evolution.
Excusing the tangent, while this may seem unbelievable to the modern reader, it must be kept in mind how little concrete information people had. Few cultures were able to agree on how many types there were; to expect people from these ancient times to seek out eighteen of Eevee's evolutions would be the height of folly when they believed there was an Earth-type rather than Rock and Ground, or that Normal-type Pokemon were immune also to Dragon-type moves instead of suspecting the existence of the Fairy-type, or even that there were three energies called Natural, Neutral, and Spiritual rather than the type system.
One must imagine the Pokemon from that last time and place being particularly frustrated with their trainers… if they knew better. As very few Pokemon are able to write or communicate in a way amenable to being physically recorded for posterity, let alone the likelihood of such records surviving thousands of years into the present, the evidence one way or the other is inconclusive!
Needless to say, even ancient Eevee lineages had little idea just how far the Eevee evolutionary family extends. Indeed, it was only in the fourth millennium, with the advent of the Paldean Empire and its attempts to circumvent trade with the Ottoman Empire, along with the subsequent fostering of the Great Exchange, globalization, and direct trade and contact between Europe, Africa, Asia, and the Americas, that humans slowly caught on to the idea that there could be an Eevee of every single type.
Though the nature of achieving Eevee's Ghost-type and Poison-type evolutions meant that many of the third-stage evolutions of those types were the last to be discovered, the last one to be discovered is currently debatable.
Eeveeluteon, Eevee's third-stage, purely Normal-type evolution, was discovered by a team in France in the early 3900's. However, recent discoveries in Alola have cast doubt onto the French's claim of first human discovery. It was confirmed that an Eevee that held an Eevium Z crystal would only evolve into its Normal-type evolutions in the 4970s, but recently discovered records on Akala Island may indicate that a Kahuna may have trained an Eeveeluteon around the mid-3500s.
At the same time, in what is today the Geesahro region, British explorers discovered a Somali trainer with the undiscovered Poison and Dragon-type evolution of the Poison-type Toxeon. The lack of evidence as to when exactly the trainer in question evolved his Pokemon, as well as the destruction of many of the primary source documents in the intervening thousand years, has led to further confusion and debate.
Finally, the current champion of Australia's Lutoria region was coincidentally sent back in time to the area and evolved their own Toxeon into Felleon while stranded in the past close to the time the Somali trainer is supposed to have done the same. While he does not claim to have discovered it 'first,' some in the scientific community pushing for wider acceptance of information gathered solely through time travel do.
In conclusion, the current consensus is that Eevee evolves eighteen times, and that each of its evolutions can also evolve eighteen times, giving the Pokemon exactly 324 third-stage evolutions. The family has exactly one Gigantimax form, one Mega Evolution, and only the first and second-stages can Mirror their trainers through Rare Abilities such as Battle Bond and Contest Connection.
Despite the current consensus, many seek out ways to push the boundary, whether to achieve fame and status for pushing the boundary or to deepen their bond with the Pokemon. It was not too long ago that a number of Eevees were evolved on the moon to determine if the species would gain a new evolution to suit its interstellar environment – they didn't; they all evolved into Sylveon, despite some of them not meeting the normal requirements to evolve into that particular evolution.
Regardless of consensus, if a new type is discovered, there is a good chance it will be because Eevees, as ever, aspire to adapt to any and all environments they find themselves in. Of course, if Eevee is not the Pokemon through which the new type is discovered, no doubt tens or hundreds of Trainers of Eevee and wild Eevee alike will push the boundaries and, undoubtedly, make their mark on history.
-OxOxO-
A/N 1: As stated in the summary, you don't need to read Battle for Peace to understand this work. For the most part, it'll be a series of unrelated 'Reports' about Pokemon, phenomena, history, and other elements of the worldbuilding I'm doing for Battle for Peace, which is itself basically an excuse for me to think about Pokemon, neat stuff I think would be cool for the franchise, and how it would or wouldn't extrapolate from a game into a world.
A/N 2: If you'd like to donate to support me monetarily, search for Sugarcane Soldier on the website of the Patrons.
Thank you to WarmasterOku, Afforess, UNSC_Kawakaze, Theewizzz, Vee, malenkaya, and Saito Tachibana for supporting this story and everything else I write. Make sure to vote if you haven't yet!