There are a number of animals that can be found roaming the wild in rust. Most animals are not hostile, however you will be attacked if you come into the range of a wolf or bear. As of the latest update, new versions of the Wolf and Bear in blood red skins now spawn in place of zombies, these "Radimals" not only drop a loot backpack, but can also be struck twice with a melee for normal animal loot
Most animals, even a wolf or bear can be approached and attacked while crouching without alerting them to your presence.
Prey animals will not notice you while crouched until you damage them, or stand up.
Predators will eventually react to your presence when you get very close to them.