Unused experience groups can be found in Pokémon Red, Blue, and Yellow as well as Pokémon Gold, Silver and Crystal.
Normal experience groups include curves hex:00 Medium-Fast, hex:03 Medium-Slow, hex:04 Fast and hex:05 Slow.
The unused experience groups are curves hex:01 and hex:02 and use the following formulae for experience at a given level, where n is the Pokémon's level.
(3/4)n^3 + 10n^2 - 30
(3/4)n^3 + 20n^2 - 70
Though unused, they may be found on glitch Pokémon. The glitch Pokémon X ゥ- xゥ, and ゥ ゥ. use experience group 0x81, which retrieves unused experience group 0x01.