Yes quite a lot of them
In the Old Chatuea in Enterna forest the non legendary Pokemon are Gastly and a very odd Gengar.You can also catch them near Solaceon Town at night
You can catch Rotom.
only magicarp
you can catch rotom next to the old cheatu on route 205
It's in the Eterna forest and you can catch Rotom there.
no you cant catch diglett on Pokemon ruby because diglett is too old he comes only back in Pokemon diamond
Placing into play that i think its the seventh gym leader CATCH UR LEGENDARY AND MOVE UR BUT TO THE NEXT GYM 10 YR OLD OWT
no not without ar ds and pokesav
you cant transfer from old games or trade from diamond.
you can catch a porygon in Pokemon diamond by going to the old man in the mansion and out to the trophy garden. You also need the national dex to catch a porygon . walk around the grass and in a little while it will appear.
upper segment of mt.coronet with an old rod
no but you can catch both Dialga and Palkiaalso you can catch articuno, zapdos, and moltres but so far i think you can only get certain Pokemon like in Diamond and pearl and the other old versions