off all the teams in the MWC (back when BYU was still in it).  They asked players, coaches, and students who their biggest rivals were.  Allmost all of the schools listed BYU as there #1 rival.  The only exceptions were CSU and Air Force.  They listed eachother as their #1 rivals, but BYU was only a couple percentage points back.

Everyone wants to be a rival to good teams.  USU is a great example.  USU has long considered BYU to be its biggest rival (correct me if I'm wrong Aggie fans), but to Cougar fans, USU was an afterthought,  just an other "Cupcake" on their schedule.  There was no antipathy, just apathy.

The attitude goes both ways though.  BYU considers Utah to be its rival (and rightfully so), but not USU, despite having a long history with both schools.  Why? The answer is that USU is viewed (whether right or wrong) as an inferior program.  BYU as a program is somewhere between G5 and P5.  Cougar fans want to identify with P5 level programs.  Having a mediocre G5 team as a rival would not set well with most fans so they continue their stance that USU is not a rival, but merely as just another middling G5 team to beat and pad their win total.