According to this map, Casco was home to Abenaki speaking members of the Wabanaki tribe including parts of the territories of Pequawket and Arosaguntacook.
History of the Wabanaki
Edited Abenaki Dictionary
An interesting fact about Casco post-European inhabitants: What we currently consider Casco, Maine was considered part of Raymond, Massachusetts until 1841.
History of Casco, Maine
The Edwards family of Edwards Road fame in Casco (near Crescent Lake) has an excellent genealogical record that goes into town history. Edwards Generations