General: slit pupils
Characters with straight or oval shaped pupils. It's a feature commonly associated with predators or nocturnal creatures, and has been used in art for a long time to make characters seem more inhuman or evil.
The most commonly known example of this is cats, but this does not apply to most felines, as most big cats (lions, panthers, tigers, etc.) have round pupils. Some species of reptiles, such as vipers (but not all snakes) and crocodiles also have those. Which animals have them or not seems to have more to do with their hunting style (usually ambushing) and their activity periods (crepuscular or nocturnal) than their biological family.