May 1836
Caleb Blodgett came exploring the site of present Beloit and returned in June; he found Thibault living there with two wives.
BeloitGrade A · 1 independent source
The evidence
“The first white man to settle within the present site of Beloit was a French Canadian trader and Indian interpreter named Thibault (Tebo), who in 1836 claimed to have been living in this general region some twelve years. There was no human habitation in this locality when John Inman and William Holmes visited it for part of a day July 19, 1835, so Thibault 's log cabin, which stood at the south end and west side of what is now State street, must have been built after that date. In May, 1836, Caleb Blodgett found him here living with two squaw wives, one about forty, with a grown-up son, and the other, a half-breed, considerably younger and the mother of a babe.”
1908-brown-rock-county/06-history-of-beloit.md, Vol. 1, PDF p. 143 / book p. 128
Brown, William Fiske, editor. Rock County, Wisconsin: A New History of Its Cities, Villages, Towns, Citizens and Varied Interests, from the Earliest Times, Up to Date. Chicago: C. F. Cooper & Co., 1908. 2 vols. archive.org/details/rockcountywiscon01brow, archive.org/details/rockcountywiscon02brow.
Grade A: Found on a page of one of our sources