What can hostile mobs attack?

Mobs that attack each other:

• Golems (iron and snow) > all hostile mobs
• all hostile mobs > you, anything that attacks them (skeleton shoots a zombie by accident zombie attacks skeleton)
• Cats > Chicken
• Wolves > Sheep
• Zombies > Villagers
• Iron Golems > any player that attacks them or nearby villagers, skeletons, zombies, and witches
• Tamed wolf > anything player attacks or is attacked by

In answer to your question, your livestock should be safe from anything but wild wolves and cats (except for splash damage from witches or creepers, or stray skeleton shots).