If you're asking whether the Titans were an older pantheon, and then the Greeks came along and painted over the wallpaper with an Olympian veneer, the answer is no. You'll want to take a look at the superb comments by /u/koine_lingua and /u/storrsh in this older thread.

It's hard to point to any major elements of Greek mythology as distinctively and solely Greek, in the sense that there are no correspondences at all in other mythologies; so whether they "invented" the Titans or not will depend on how you draw the line around what "inventing" means. Zeus is a very Greek deity, but it's easy to find cognates to him in other mythologies. Does that mean the Greeks didn't "invent" him? A moot point. Is Zeus a Greek deity, then? The sane answer should of course be yes. The same line of thinking applies to the Titans, who also have various cognates.