In the English dub they call them Bounts and not Bounto, which is pretty close. Anyway, the Bounts while not completely explained are said to be born and are unable to give birth themselves. So they make me think of them being like the immortals in Highlander. They are born by normal human beings and just a random genetic anomaly. However, they did say that only a certain number of Bounts can exist. So it seems that when one dies somewhere else another is born to maintain the balance or something like that.

As for Kenpachi like Shadow said there is not really a reason given for him. He just simply has a tremendous amount of spirit energy beyond any one else. It is the only thing that allows him to be at the level of a captain with the boosting from a released zanpukto.