What is the frequency of raids?

Rhydon was actually the first pokemon ever created way back when. If you played the main series games there were significantly less sprites for your party's screen. Originally the idea was to have all pokemon resemble Dinosaurs but that got thrown out. All the mons that do resemble dinosaurs received the basic Rhydon sprite. In fact throughout the main games there are statues all over the place in gyms and to the elite four. They are all Rhydon. I'm guessing this is just a way to pay homage.