I would think that the Thunderbolt at Kennywood would have the claim as being one of the first terrain coasters.
Actually that would have been the Pippin. The Thunderbolt is relatively a new thing, composed, in part, of Pippin.
In any event, right chain, wrong coaster.
Think Rollo Coaster at Idlewild. Obviously a terrain coaster.
http://www.rcdb.com/ig272.htm?picture=10
And there are many others, too..
Saves on the building expense, you know.
Even Son of Beast takes tremendous advantage of the terrain it is on, or it would be even larger! It would have taken far more wood to build, were it not for the terrain. I remember what a big deal Jeff Seibert made of this at the time.