I cannot tell how high up this is from the ground. The most notorious animal that feeds on arborvitae are deer. They can strip the lower portion up about 4 feet. I have even seen them standing on their hind legs feeding on plants. The picture makes it look like this damage is higher than 4 feet. Squirrels feed on many plants in early spring but not aware of this happening to an arborvitae. It is also strange that is only happening to the end plant though it does happen where it is the most accessible. I do not know of other animals that damage arborvitae.