When Ho-oh dwelt in Ecruteak's Brass Tower, there were no such thing as Fearow. Spearow existed, and they were a violent pokemon which Ho-oh hated for driving humans to war with wild pokemon, but could not bring itself to destroy; it seemed time would do so anyway, for their numbers were diminishing by the day.
And then the tower burned down, attacked by an angry, power-hungry mob. Ho-Oh raised the legendary beasts from the ashes of the tower, then flew away to a Spearow flock which it reshaped in its image. The pokemon grew in size, their necks grew long, their beaks longer still, and their heads and wings changed shape into one vaguely reminiscent of the legendary bird itself. They gained a few new attacks, drill peck the most powerful, and were gifted with the power of true flight – not of moving from tree to tree for a few hours like "flying" pokemon do, but of staying aloft forever like the gods themselves, lest they be attacked in their nests while they sleep.
To this day, when a wild Spearow becomes powerful enough, they seek out Ho-oh to evolve and join the original flock of Fearow which bring about the fire bird's wrath to this day. A similar technique using ash from the burned tower has been developed to create trained Fearow, but they never seem quite as powerful as the ones in the wild: they are Pidgeot with less speed and longer beaks, not a terror from the skies sent by an angry god.
