Buneary, as a species, is descended from a parent species of Pokemon that has been made extinct by the first settlers of Sinnoh.

The parent species were friendly in nature, otherwise they wouldn’t have let humans get close to them, but that proved to be a mistake. Most of parent species were captured, the wild stock were deliberately killed off by strategic use of agriculture and domesticated predators. The enslaved survivors were then subjugated to the cruel hands of the Sinnoh settlers, and since Pokemon are such emotionally-driven creatures, that caused an oddity. Those whose spirits were broken managed to gain an obedient and affectionate evolution, a logical step in the grooming process the settlers put the poor creatures through, while some of the rebellious stock managed to break free and escape to the wild once more.

This, ultimately, lead to the creation of Buneary and Lopunny. The former, sensitive and alert, detests humans and distrusts them. The latter, an aloof victim, loves the very creatures that put it through intense and calculated pain, all for the sake of primal pleasure.