I know a lot of people are going to disagree with this but rend resistance is something that doesn't need to exist, even on cunnings, much less fierces and resilients for several reasons. Also before I even start I will address thylos, all it needs is to have dual rending assault replaced with rending takedown or rending attack or maybe reduce the rend on each hit to 25% or 30% and it's no longer going to be super broken because of rend.
- Rend is NOT a negative effect
Unlike any other effect, including bleed, rend is just straight up damage dealt to an opponent with no extra effects added on outside of a few moves like defensive rending attack. You'll notice that if you tap on another creature you won't find "rend by x% for 1 turn" like you would for literally any other negative or positive effect, if you use lethal wound against a bleed immune creature you still do damage because it's a strike with an effect added on but if you use rending takedown on a rend immune creature you will do nothing because it's a basic damaging move, almost like using rampage on a damage immune creature. Rend clearly takes the place of damage in any move it's used, in defensive rending attack you don't deal damage alongside rend as if it were like bleed, you rend and shield instead so rend is just an alternative form of damage
- There's other ways to kill resilients
This is one of the worst arguments for giving something not even remotely cunning rend immunity because it's basically saying every rend specialized creature essentially deserves to be forgotten about and made worthless for no real reason. I also have to say in that case, there's other ways to reduce damage so let's have 75% distraction resistant thylos, distraction immune aeno, rexy with 67% distraction resistance because who cares about cunnings that don't dodge? Or why not give dracovenator, CG, and dilorach vulnerable immunity since resilients can just use shields and dodge removal to kill them?