Branched evolution is not unknown to the pokemon world. Even in Kanto, Slowpoke and Gloom can evolve in multiple ways. But these lines involve two evolutions, while Eevee has eight known as of this writing, with more likely to be discovered in the coming years.
Eevee has therefore been the subject of intense research, most of it funded in the hopes of a grand discovery of how pokemon evolution works. Team Rocket in particular funded studies so intense that the wild Eevee in the region were virtually all turned into lab rats for experiments, as Giovanni hoped to unlock Eevee's secret and use it to create powerful armies of evolved pokemon which had never been seen before. At this task he failed completely, but his creation of an evolution-changing Eevee led to some answers, and many more questions as to just what this pokemon was. Theories abounded comparing it to the formes of Rotom, or the myriad of Unown shapes, but none of them could account for why this could have developed into a permanent change.
Today, as further experimentation has discovered little, most of the Eevee captured by Team Rocket have been sold off, some for the power of its evolutions, most because their appearance made them coveted as pets. Some have been murdered, as their strange evolutionary prowess has led to a surprisingly large number of people considering them a crime against nature which must be extinguished. Sadder still, many have resorted to abusing their Eevee to prevent them from gaining the happiness needed to evolve, citing the difficulty of caring for their evolutions or simply a desire to keep them cute and fluffy forever.