Lava World

Mount Fire Eyes was a unfinished world that was planned to be released in Banjo-Kazooie. Due time constraints, however, Rareware did not feature the level in the game along with Fungus Forest, Giant's Lair, and Hammerhead Beach. Instead, the level was reused as the lava side of Hailfire Peaks in Banjo-Tooie.

Like Fungus Forest, there is a direct reference to this level in Banjo-Kazooie in Click Clock Wood, as after stealing Gobi's water for a second time he will state that he is going to go to the 'Lava World'. Although in Banjo-Tooie it turns out he was heading for Hailfire Peaks, it is presumed that in the original game he had meant he was heading to Mount Fire Eyes, which would have been a tenth world after Click Clock Wood and would have probably been in the small lava room where Mad exists, maybe where Mad Monster Mansion exists in the final product.

Mount Fire Eyes and Giant's Lair Video
Here is some footage of gameplay on Mount Fire Eyes on E3 before the game was released. Mount Fire Eyes / Giant's Lair