While using binoculars to look into a town 800> meters away I noticed a zombie in a field chasing something, despite it being an open field I couldn't see what it was following. I then noticed several cows in the field, but with closer observation I noticed that these cows were completely frozen, no movement of any kind, some seemed to be frozen in a walking animation.

My theory is that at long range (800 meters and above), cows and possibly other animals don't appear to move client side but are still moving on the server.