Today, when we think about animals, we think of pets. Even if we don't have a pet, we tend to have a favorite type of animal. An animal that we associate with or relate to in some way. They bring us joy and comfort. Further more, we don't like to think about death, let alone the death of this creature.
Yet when we have meat for dinner every night, we don't think of it as having been an animal that brought about comfort and joy to our world. It is simply food for our bellies.
People that have the experience of living and working on a farm or ranch make a living of raising animals for meat. They too feel the joy and comfort that pets can provide, but know the neccesity of killing for food.
(Every vegetarian would disagree that it is a neccesity. Where there vegetarians back then?)
And these farmers/ranchers kill animals daily. Perhaps this is why they use the term 'livestock'...
It is an impersonal act to kill 'livestock' as apposed to killing your most favorite, your most prized sheep. (As modern city dwellers, we might be better off thinking of this prized sheep as our precious pet.) This is what GOD requested for sacrifice. Our most prized, most favorite, most costly to lose, most important, animal.
I think that this sacrifice would be difficult for even the rancher who is used to killing livestock everyday, let alone a city dweller.