As a combination of these two questions, are the unforgivable curses unforgivable in all cases, or only when used against humans (muggles and wizards)? If, as example, a wizard with low regard for non-human life were to use the cruciatus curse on their house elf, would they be bound for Azkaban?
There's definitely some distinction between animals and humans as Barty Crouch Jr (as Alastor Moody) demonstrated the Unforgivable Curses on spiders with no repercussions. Given you specifically mention house elves, I'm guessing you're more interested in non-human sentient life, in which case Crouch also says "The use of any one of them on a fellow human being is enough to earn a life sentence in Azkaban". Given the wizards' opinions on house elves and other non-humans, it is probably not considered unforgivable to torture your house elf with the curses if you're so inclined.