According to wiki

The name Ontario is likely derived from the Iroquoian word Kaniatarí:io or Oniatarí:io meaning “lake of shining waters” or “beautiful lake”.[10] It has been in use since at least 1639.[11] During French colonial Canada, various other names for the lake were also used, including Frontenac and St. Louis, though Ontario ultimately became the common term.[11]

So a rare use of the given name by the local people. The french had a go of a rebrand but it didn’t stick.