Boxer is a hard-working, dedicated shire-horse. His enormous physical strength and courage come in use during the epic Battle of the Cowshed, where he plays a leading role in driving the hated human oppressor from the farm. Unfortunately, Boxer's not exactly the sharpest tool in the shed. Unlike the pigs, his strengths are physical, not intellectual.
This makes it all too easy for the pigs to manipulate Boxer, along with all the other less intelligent creatures on the farm. Boxer passionately believes in Animalism and the success of the revolution. But he's too naive and too ignorant to see that Napoleon and his gang are subverting the very principles of the Animalist revolution to serve their own ends. This makes him especially suggestible to Squealer's outrageous lies and propaganda. Whenever something goes wrong on the farm—as it frequently does—Boxer instinctively goes along with the party line, blaming the farm's problems on convenient scapegoats, such as Snowball. As far as Boxer's concerned, Animalism is always right, so if there are any problems, then it must be due to deliberate sabotage.
Boxer's naivety eventually lands him a one-way ticket to the glue factory. His tragic fate is an allegory for how the workers were treated in the Soviet Union. This was supposed to be a worker's state, a worker's paradise, no less, and yet the workers were systematically enslaved and exploited by a regime that was supposed to serve their interests.