Rhyhorn 100%

I think Mud slap / Stone edge is the best Rhydon moveset for attack as well as for defence (if you ever use it for defence). So, YES, definitely worth powering up!
I keep mine (13,15,15) powered up all the time and is the one i use to wipe out Snorlax, Dragonite, Gyarados, and whoever else stand on the way!

You should probably transfer it the ivs are too low

Do you mean Rhydon or Rhyhorn?

Depends on what else you have.

For defense, no matter the moveset, it's a "bad defender", but since it has high CP, it will sit high in gyms. Having said that if you have a snorlax or a blissey those too will sit high in gyms AND are actually good defenders.

As an attacker, level 26 is already high enough that it will defeat most pokemon you want to pair up against him with some dodging and picking the right match ups, plus it having 2 different move types makes him really only good against fire (the only shared strenght) type pokemon, which is not a very prevalent type these days.

I wouldn't power him up, unless I have nothing better (Snorlax, Blissey and Dragonite to a lesser extent) to put in a gym.

Not sure why you're asking this. If you didn't power up a 100% Rhydon with a great moveset, then what would you power up? Not much.

The answer is the most obvious "yes" this site has ever seen.

If you are not concerned about Niantic nerfing Rhydon and / or adjusting its moves in the future, then there is no reason not to power it up.

Do not power it up unless you evolve it and see if the Rydon have a good moveset!!

If the evolved form have a good moveset, then you power it up as you may power up that Ryhorn and evolve it and end up with a Rocksmash/Megahorn Rydon and it will be a total waste.