Here's a nifty one I found out last night.
The Japanese/American movie King Kong Escapes is loosely based of the 1960's Japanese/American cartoon series The King Kong Show, as it features MechaniKong and Dr. Who (not that one), both who came out of the animated series. And both were co-productions of Rankin Bass.
Additionally, King Kong Vs. Godzilla was almost King Kong vs Frankenstein and had it been made, it would have been an American only movie.