golem?!

I like Golem as both a defender and attacker. I have one with Rock throw and legacy ancient power and I do use him as a secondary defender. Still gets wrecked by water, but pretty much everything gets wrecked by something.

Golem has the same weakness as Rhydon (double SE to grass and water) but with almost 400 less max CP, which can be a big deal for some players in using it as a defender. Rock Blast is a good defending move but Golem still has 50 less base stamina than Rhydon, which results in about 37 less HP (74 on defence), which I think outweighs the benefits of Rock Blast, plus with Rhydon sitting higher it'll be harder to get shaved off gyms, which is mostly how I lose my gyms, but maybe your situation is different.

Golem's niche it has over a Rhydon is that it can have a double Rock type moveset. Mud Slap Golems are just worse Rhydons, whereas Golems with double Rock moves have a small niche against flying type Pokemon.

Rock Blast is pretty good on attack in my opinion, because I prefer being able to dodge every move, Rock Blast is often fast enough to use even between quick moves, so you don't waste any energy. I'd rank it above Ancient Power for that reason.

The only good thing of rhydon and golem as defenders is their considerably hight CP, both are ok but have 2 double weakness. RT is not a great defending move, even tough RB is better than Eq or SE, you don't have such a great moveset for defense, so go for rhydon CP. He could really work as a attacker though.