Then added WSU and Stanford in 1916 and 1918, respectively. USC and Idaho were added in 22, Montana in 24, and UCLA in 28. That was still the PCC, however, not the PAC 8 (there were 10 schools at that point, guess who gets left out when the Pac 8 gets formed in the 60s).

Oregon, OSU, WSU and Idaho left the conference in 1959 because of scandals in some of the other schools (Montana was already out by this point). Oregon, OSU and WSU all came back within 5 years, but not Idaho. The designation "Pac 8" was then used starting in 64.

So yes, Idaho was part of the conference that would become the Pac 8 (as was Montana), but not while it was called the Pac 8.