Rhydon evolve

Difficult question, but in my opinion, wait for another one (maybe from egg)

Honestly 31 IV is crap. Wait for a better Ryhorn

The only reason you would really want a rhydon is to bust yo snorlaxes in gyms. I'm sure, like most places, gyms in your area have a fair number of them so you are looking to acquire a quick solution. It looks like what you have is a short term vs long term solution dilemma on your hands. So some quick questions below that might help you in your decision:

1) are you currently having lots of problems with the snorlax problem or have you been able to get by so far?

2) is there a buddy that you are currently working on that keeps you from walking more rhyhorm candies off?

If you absolutely needed that snorlax buster yesterday, then I guess you have no choice but to go for the quick solution and evolve the lvl 27 and power it up and go at it. If you have a way now of getting by ( new ones are sldo being added everyday, btw... the new jacked machamp moves as an example) , I agree with the reply above and say that you should wait for the hatch or a rhyhirn nest to cycle your way.

Another reason you want to wait on the hatch is that there is no guarantee that the level 11 will come sans megahorn (not putrid but def the twister of rhydons.) at 87 candies, a hatch will give you enough additional candies for two cracks at a good iv rhydon with good moves. If you cAn continue to walk more candies, then you are also in a position to jack either of them up if you get the right moves...

My three cents

OMG do NOT evolve a 31% Rhyhorn. He's best in gyms and in the long run you'll regret missing out on a couple hundred CP. Trust me!!!!

wait to hatch one with a higher pokemon lv.

If you want it to place high in gyms as a defender, then you'll want to get all the cp possible. If that's the case don't evolve a low IV rhydon. It'll cost you a significant amount of cp.. enough to mean the difference between top of gym or bottom. Whether you evolve your little 91% guy depends on how much candy and stardust you are willing to part with. Otherwise .. patience.

Wait for a better IV if you want to use for Gym defense.

For att it doesn't matter quite as much.

By the way, Rhydon is not the only counter to Snorlax. Yes, he is the best counter with RS+EQ/SE but there are plenty of other pokemon out there who will do pretty much the same job with the right att moveset at the right level. (Vaporeon, Dragonite, Machamp, Golem, Ursaring, Archanine, Nido Q / K, some other gen 2 I am sure I'm missing... etc)