Groudon, Raquaza, Latios/Latias, Lugia/Ho-oh, Cellebi, Deoxys, Jirachi, Mew, Mewtwo.
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
raikou cannot be caught on Pokemon ruby
...and?
Solrock is not such a rare Pokemon but you can only get them in ruby/emerald
Catch Pokemon you already have caught
You have to migrate it from Pokemon Ruby, Sapphire, Emerald, or any other Pokemon game that you happen to have a Torchic in. Torchic can't be caught in Pokemon Diamond or Pearl. Sorry. You have to migrate it from Pokemon Ruby, Sapphire, Emerald, or any other Pokemon game that you happen to have a Torchic in. Torchic can't be caught in Pokemon Diamond or Pearl. Sorry.
Illumise in Pokemon Omega Ruby can be caught in Zorua at the park.
You catch them. Or trade for them.
n e where
you can migrate any you caught on ruby,
Rare Pokemon come randomly when you're in the grass or cave.