What to do with 500 rare candies?

Do you still need to complete your pokedex?

For legendaries, Moltres and Zapdos are the most worth candies. But I won't give legendaries any rare candies.

Maybe just divide a couple hundred up among useful Pokemon going forward, like Dragonite and Tyranitar. I know that might not be ideal, but at 300 rare candies I am feeling the bag crunch.

Convert into pidgey candy, put down a lucky egg, mass evolve session, enjoy xD.

Just a joke, if you don't know what to do with them probably the best choice is just save for the future. Zapdos will be here soon and you would probably want to max one of them out.

Also another reasonable option is stop paying that much money or using that much of premium passes. You must be a good spender in order to have that amount of rare candies so why keep spending more if you don't even know on what to invest at this point? Or just buy incubators to generate more dust, could be another option, in order to power up another mon along with those rare candies.

Look into Gen4 pokemon evolvutions and put some rare candy into things like Togekiss and Tangela if you dont have enough to evolve some of those

I would wait at least one more week, maybe spending some excess candies for rare Pokemon that get an evolution in Gen 4 such as Porygon 2, Tangela, maybe Rhydon, Sneasel, Lickitung, Togetic
and hope for Mewtwo to hit the gyms soon.

Ho-Oh and Mewtwo appeared in the legendary teaser, so Mewtwo could appear once Zapdos week is over

Ho-oh will be a good use for candy in the near future. Its maximum CP is 4.6k you'll require a lot in order to reach that 4k range.
Even though it doesn't benefit from quick attack stab, it's certainly going to contend for the best DPS in the game.

1. If you do not have 6 level 30 Golem, then I will use rare candy to get that line up. This is for Zapdos. The counter argument for this is; geodude candies are easy to get as buddy candies. However, having 6 level 30 Golem is a lot of candies (if you are leveling them from level 20 egg hatches)
2. Use it to power up your Piloswine. Again Zapdos. Plus it will not be wasted, as its third level evolution Mamoswine will be an absolute beast when that is available.

I just caught a 14/15/14 Moltres, makes me feel like I made the right decision about never putting a grain of dust into a Flareon nor a Charizard

Besides the above mentioned, why not Blissey? Still annoying to fight her.

Did you maxed your bag?

GeodudeW/Golem is the cheapest candy to walk at 1 candy/1 km. Legendaries are 1 candy/20 km. So this is your best use IF you want to power one up. Next best is draggy and TTar at 1 candy/5 km and hard to find in the wild in most biomes. I would put it to use for what you need now as it seems easy to obtain unless they change the raid rewards.