Conor McGregor and Khabib Nurmagomedov provide perhaps the best example of a visceral rivalry in mixed martial arts where the animosity between the two individuals involved runs so hot that they cannot even be in the same physical space with each other. Renzo Gracie and Wallid Ismail hold that distinction in Brazilian jiu-jitsu. Their rivalry has lasted for three decades.
However, the situation appears to have changed, at least from Ismail’s standpoint.