In the beginning, it just the god of man and the god of beasts. While they were in agreement in creating a planet, they couldn't agree what would be more dominant species on the planet. After hundreds of years fighting they finally agreed to let those on the planet decide what they want to be. So all over the world, they placed trees that would produce two different fruits that would be eaten on their 18th birthday. One that would keep them human and bring them to the peak of manhood and another that would turn them into a random anthro beast.
The beings of this world do not wear shirts of any kind and until they eat one of the two fruits, they would only wear shorts of some kind. Around their 13th birthday, that is when their pecs and one random body part would develop a bit. The random part would be an indication of what anthro they would be if they were to eat the fruit of the beast. For example, if their legs develop more there's a good chance they'll be an anthro cheetah.
Upon eating the fruit, that is also when they have a sexual awakening and experience their first erection. Depending on the fruit, either an M or a B with be on their chest for two weeks. Which is how long the transformation will take full effect. It always starts from penis and moves outward to the legs, chest and back, arms and finally the head. Once the transformation is completed, they now have the ability to bear children.