Well, I must say. I can't describe the logic behind everything that Pokemon does. However, I actually came up with a reason as to why Rhydon can learn Surf a while ago.
Rhydons horn is reportedly harder than diamond.(Stated in several poke dex entries) How is something that strong created (keep in mind, rhyhorns is not reported to be as strong)? Only immense heat, probably DEEP in the earths crust. Maybe rhyhorns that used to be in caves, or even land, had something environmentally drastic happen, such as an earthquake that made them fall into the earths crust. How did they deal with this drastic event? Evolve. They evolved, to something that now has, not a horn, but a drill. My guess is that the earthquake caused a tsunami(s), obviously flooding the lands, by evolving because of this event, rhydon can now learn surf, and has a drill. Rhydon is said to beable to withstand heats of up to 3,600 degrees. (Stated in several poke dex entries) Which is conveniently a sustainable temp to survive the heat needed to create a material harder than diamond.
May not be right, but better than Pokemon logic
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