Tyrannitar is one of the exclusive legendaries. The pokemon line itself is in Y but the stone for it is in X.
That sucks about ditto.
You can actually get into Pokemon Village before post game though, its accessable just before the eigth gym, in fact you find the gym leader there. Ditto is only unaccessable before post game if its in the cave. I cant say where it is as I havent gone hunting for one yet personally. I should though, I really want to make a lv 1 Larvitar to train up so that I can add a Tyranitar (my favorite pokemon) to my party.
Also a few other things that are bugging me as I play:
When you use healing items in your bag, it automatically assumes you want to use another of the same type, and you actually have to cancel it, or else you are stuck in a healing loop.
Rock smash things everywhere. My god on the right half of this world it seems like you need rock smash to get anywhere remotely good, which would be okay, if it wasnt such a useless combat move because its so weak. One thing that they really should fix.