Wild Boy (ワイルド ボーイ)
According to unMAMEd, it was announced in a Taito newsletter, but there isn't any evidence that it was ever released. Prototype?
The flyer for Wild Boy is pretty sparse (no screenshots, perhaps from some Taito newsletter). It describes the game as a Spartan X-type game that could also be seen as an upgraded version of Jungle King where you could walk on elephants.
Related to Jungle Boy?