Growing up a TCU fan, I never considered Baylor a "rival." They were just another school. You'd hear weird stories about them occasionally, like not allowing dancing, etc. But they were bad at everything. Course, TCU wasn't great either. But they were just a small, weird, (forgive me) gay school in a terrible little strip mall town you drive through when headed south on 35.
I think the Big 12 snub was what started it. People understood getting left out for UT and A&M, with those two kind of being the State flagship schools. Tech sucked for other reasons (us getting THEM into the SWC when they were basically Abilene Christian). But Baylor was such a chump school and the politics of it played out in front of everyone's eyes and it just pissed us off. Then, came the superior attitudes that they would have for no good reason other than being in the Big 12. (Obviously this was a tech affliction as well)
Course it all kept growing, as others have detailed. Eventually, when we get in the Big 12, and have multiple BCS bowls in our recent history, a Rose Bowl win, numerous 10 win seasons, etc....and they have like 2 or 3 years of 7 man success with a gimmick offense, and they start to talk and act like their success was not only on par with ours...but that their program had far surpassed us.
Of course, it was all a flash in the pan. All built on rape, and classless play on the field, and steroid use (yep). Karma is awesome. Rape boy and GP had some great battles, on and off the field though. The hate between them was real and it definitely fed the "rivalry." Obviously 61-58, and all the nonsense about that. Its to the point that I do believe it is a rilvary and I absolutely despise that program and that school.
But I was always surprised by the obsession on this forum by some with all things Baylor. I kind of would just like to go about hating them like I hate Tech or Aggie or Horn. I think it legitimizes them to have them be our "rival." When it was us vs them for the CFP spot, I always thought that their program benefited from being associated with ours. Like part of our journey and success since 2000 had rubbed off on them when people couldn't mention TCU without mentioning Baylor.