There are DeCoteau's in my family. I had an aunt Alice DeCoteau and she had a brother Peter. I know they spell their name differently but that could be attributed to Americanizing the name.
Legend was that a D'Argis male was married to a DeCoteau, who possibly came from Indian stock. There may be some merit to that as there is a large tribe of Chippewa in North Dakota, on the Canadian border, with the last name of DeCoteau.