I dislike most a whole heck of a lot of teams, but most of those are just because they beat my team at some point or another. And then there are the teams that used to be teams I rooted for that slowly turned me off (notably the Dallas Cowboys, Texas Rangers, Dallas Stars, and the University of Louisville).
My raging hate is saved for two entities and two only: Texas A&M and Baylor.
I hate Baylor largely because my mom hated Baylor and then grew up listening to Baylor propaganda spouted by my grandmother who was a die-hard Baylor alum (my dad and grandfather were also alum, but not that into sports so they really didn't care. My uncle is also an alum and he barely ever mentions Baylor athletics, well, except for a few years around 2014 when he talked smack).
Mom swears up and down that when Coach Pittman died on the sidelines during the '71 game in Waco, that the Baylor band started playing their fight song after his death was announced to the stadium. She wasn't even a TCU fan at the time but that was enough to make her hate Baylor.
The Aggies were another team mom hated but I had my own run-ins with the agholes. First was when I was 10. Aggies at TCU, the craptacular 1985 edition at AGC. I hadn't yet realized just how awful humans could be and so my night was ruined, not so much because of the score (although that didn't help) but because of the two agholes sitting directly behind us that had smuggled in booze, got drunk, and then shouted obscenities about Coach Wacker. For a 10 year old that had met Coach Wacker and gotten his autograph, this was painful.
Second time was when I was parking cars for University Christian Church to help raise money for some of the youth groups. Traffic was atrocious (I believe it was the 1991 game) and so I was trying to direct traffic. Standing in the street and having cars take their turns to get to the parking lot, one aghole in his Suburban just didn't want to wait his turn and kept driving (slowly) until his bumper hit my leg. Literally ran into me. If I hadn't been a scrawny teenager, I might have pulled him out of the car. Still kinda wish I had.