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I've got Eelektross in my dex, so that isn't the issue. I think there is some weird connectivity issue. I've just power cycled my ds, so last try before I throw my hands up :)
Bizarrely it's because it was sniped. Somone else sent me a Groudon for a lv 1 Magikarp!?! And since I just wanted it for my Pokedex, please distribute to someone else in need! Bizarre.
EDIT: A shiny Groudon. Not a penta, but perhaps Christmas miracle?
I think what is going to be really interesting is Gen 7 - or whatever else they decide to make of Gen 6 - seeing as how GF intentionally made it so the National Dex could be completed between XY and ORAS. To me it says that GF has decided to, more or less, make everything previous intentionally...
Do you still need a Snorunt? I have a 5IV Timid Female with HA you can have - the IV's in are in the wrong spot, so you might want to breed to get them in a different spot.