I would love the ability in the config files to have passive mobs like Cows, Sheep, Pigs, etc, become Infernal.
From what I can tell by a cursory glance in the config files and some of the source, it seems like it is explicitly disabled.

I know I found this line in InfernalMobsCore.java that explicitly prevents tamable mobs like wolves and horses to become infernal. It's also possible I'm reading it completely incorrectly.

private boolean isClassAllowed(LivingEntity entity) {
        if (entity instanceof TamableAnimal) {
            return false;
        }

Unfortunately I do not know enough coding (or even any minecraft mod coding) to be able to submit a pull request for that feature.

As far i recall, i did try this once (Infernal Wolves sounded awesome) but quickly crashed into reality: The wolves "target" their owner when following them or even looking at them, same as they target things to attack. This targeting is enough to trigger several infernal effects.

Infernal Mobs would need a bunch of new code figuring out "am i allied to my current target?" before popping effects. It is not impossible but i don't wish to invest that time, as of now.

If we don't do TamableAnimals, could we at least allow Cows, Pigs, Sheep (farm animals) to be infernal? Or do all those fall under the same entity class?