I found a Harms Family Tree on Ancestry which contains the Hunter family. I asked the owner to contact you.
I'd contact the Cassia County Museum and they appear to be connected to their genealogy society too. Go to cassiacounty.org/historical-society/museum.htm .
Also go to your local library and ask about books regarding Cassia County. If Hyrum helped found the county, his picture should appear in an early book about the county. Then ask your library to do an inter-library loan on the book.
You could try to trace Hyrum's descendants. Request a copy of Hyrum's obituary from Random Acts Genealogical Kindness. Go to raogk.org/, read guidelines, choose Idaho, choose Cassia Co. and choose one volunteer.
He had one daughter Elsie Elzina Hunter who died in 1990 but it doesn't appear that she married. A Wilford Elmo Hunter died in 1978. Successful Searching!