The rheryu were born of epic magic, bred to protector their creator and their charges. In their first generation they were the watchful guardians of the kopiko. As they and the kopiko spread to the material plane from their home demiplane of origin the rheryu have spread onto protecting other things. The idea of protecting seems to be almost genetic at times, and it may be that the dragon blood within them has taken a dragon's love of treasure, and applied that love to living things.