Groudon is rare and you get him just by following the story line. The only other "rare" Pokemon I'm aware of in Ruby would be Relicanth, a rock/water type fish Pokemon that can be found underwater.
You can get three rare candies in Pokemon ruby sapphire and emerald
...and?
Solrock is not such a rare Pokemon but you can only get them in ruby/emerald
n e where
You catch them. Or trade for them.
Rare Pokemon come randomly when you're in the grass or cave.
use the itemfinder
There are a few rare Pokemon you can get in the safari zone they are pinsir, wobuffet and girafarig.
i think there's none
Pokemon ruby sapphire and emerald
you can get feebas on route 119 very rare
1d10t