Aang's past life from the Earth Kingdom is Avatar Kyoshi.
Kyoshi was an Earth Kingdom Avatar who lived generations before Aang. She was born into a turbulent era and became known for her imposing stature, formidable earthbending, and uncompromising approach to justice. Her traditional golden war fans later inspired the weapons and style of the Kyoshi Warriors.
Kyoshi is remembered for several major historical acts. She separated Kyoshi Island from the mainland to stop the warlord Chin the Conqueror, an event that caused Chin's death and shaped Earth Kingdom history. She also helped establish the Dai Li, originally an organization intended to preserve Ba Sing Se's cultural heritage.
The Avatar cycle moves through the elements in a fixed order. Kyoshi was followed by the Water Avatar Kuruk, then the Air Avatar Yangchen, then the Fire Avatar Roku, and finally Aang. That is why Wan, the first Avatar, is not the answer: he lived far earlier. Yangchen and Kuruk were also Aang's past lives, but only Kyoshi came from the Earth Kingdom.