From MangaHelpers:
In the year 2020, it was found out that a harmful virus had infected the human race, killing them slowly and bringing them towards a dark future. But with the technology they had in hand, in an attempt to save the civilized race, the people chose to go through with "Beast Transformation." Knowing that other than humans, all animals were safe from the virus, the humans made the decision to inject the genes of animals into themselves to become part-human, part-animal. In the end, the idea was "Either you do beast-transformation or die." With the race known as Homo Sapiens gone, by the year 2035, being a beast-human was normal.

In this new world, one strange boy, Kokuryuu Kazuto, 14 years old, is the worst runner and least athletic, but somehow also the most popular, with girls giving him hugs and boxed lunches, to his chagrin. But one day, he's approached by three girls, members of a group called the "Pure-bred Special Ward," who claim that Kazuto is actually a human, who should not even exist. Although he's in disbelief, he has to accept it as fact when he finds out his "ears" and "tail" are all fake. How will Kazuto face this dilemma, when he's a human in this beast-human driven world?