Rodan (unreleased NES game; late 1980s-early 1990s)
Included with the 1988/89 NES game Godzilla: Monster of Monsters was an ad for a game simply titled Rodan, based on the titular Pteranodon-like flying monster, who was an ally/foe to Godzilla and even starred in his own film. It was also briefly mentioned in several magazines, such as issue 16 of Nintendo Power, as well as the September 1990 issue of GamePro, and was said to be a direct sequel to Godzilla: Monster of Monsters.
The game was merely cited as featuring "Rodan, Godzilla's arch enemy" (despite King Ghidorah fitting that title more). It was advertised as "coming 1991", but the game was cancelled for unknown reasons. Rodan did, however, appear in what became the actual sequel, Godzilla 2: War of the Monsters. It's unknown how far into production the game was or if any prototypes exist.