Ethiopia is the birthplace of coffee: it is in the forests of the Kaffa region that Coffea Arabica grew wild. Coffee is "Bun" or "Buna" in Ethiopia, so Coffee Bean is quite possibly a poor anglicized interpretation of "Kaffa Bun". We consider Ethiopian coffees to be some of the best in the world, and extreme genetic diversity of the coffee shrub is certainly part of the reason why. Most of the coffee is either wet-processed - resulting in a vividly bright cup, with fruit and floral notes - or dry-processed with the fruit skin intact. The latter technique produces a very different, rustic fruited flavor profile, and with thicker body.
Coffea Arabica was also found in the Harar region quite early, either brought from the Kaffa forests or from closer areas around the Sudan border. It is entirely possible that slaves taken from the forests chewed coffee cherry and spit out the seeds, thus spreading it into the Harar region, through which the Muslim slave trade route passed.