The International Submarine Band. The first notable band Gram Parsons was in, before he joined The Byrds and contributed to their Sweetheart Of The Rodeo album, Safe At Home was probably the first country rock album.
After leaving The Byrds, Parsons and Chris Hillman formed the also less well known Flying Burrito Brothers, fusing country rock with soul in what Parsons called “Cosmic American Music”, which could be considered the forerunner of modern Americana music.
The Byrds themselves can’t really be considered lesser-known, but there is a great deal more to them than their hits, and they influenced a lot of people.
“Safe At Home”, their only album, is not only arguably the first country rock album, but one of the best country rock albums ever. “Cosmic American Music” was all there from the beginning.
ETA: just want to add how much I appreciate your post. Country rock is a fascinating subject, and the Byrds were the nucleus and catalyst of the genre.
I had never heard of a blues guitarist named Clarence White until someone donated some of his songbooks to the library where I volunteer. We list things that aren’t suitable for the self-service bookstore on Amazon, and within the past few days, one of them sold for $200, so he must be quite well-respected.
Yes, Clarence White is well-respected, but not as blues guitarist, but first as an excellent acoustic bluegrass and later electric country rock guitarist, in exactly the country rock scene centering around the Byrds (which he made early session work for and joined later) that were the subject of the two latest posts above.
Uncle Tupelo was a massive influence on alt-country and Americana music. The two main forces behind it were Jay Farar and Jeff Tweedy. Tweedy went on to form Wilco, while Farar formed Son Volt. Uncle Tupelo is hardly known outside of alt-country fans, but hugely influential.