Meowth is even harder. Not available in Hoenn, but the 1st Gen. remakes spammed it. Soul Silver also had it, but NOT Heart Gold. Not in 5th gen. except for a Yancy trade and maybe one more (though I think I accidentally sent a Shiny born in Unova to Kalos without cloning it because I never noticed...DOH). Not in 6th gen. at all. I recently bred some and sent them in WT (one got me a "NICE" afterwards); even received a different one this evening. That Shiny? Now a Lv.61 Persian and not losing track of it again.
Rattata, at you noted, is a harder one too. It's the first in Pokédex order that cannot be caught in any form in any way in Kalos, but must be imported. Meowth is second lowest. There are a couple more, and I remember Johto is missing Stantler in Kalos among others.