Rhydon is called a softcounter to snorlax, because it has enormous stats and resists two of its three charged attacks.
When presiging, those stats are irrelevant, as you have to use a low-CP-PKM. In addition, for prestiging you have to dodge a lot and Rhydon with its slow quick-attack is not very good for dodging.
I don't expect rhydon to be a good prestiger against snorlax - but i would be glad to read, if someone has tried it.