Certainly, Boxer does not hurt the animals as bad as Jones or Napoleon.  Boxer is nowhere near this level of hurt to the other animals.  He is the model of work ethic and obedience, someone who supports the other animals' efforts in the pursuit of Animalism.  Whenever the farm is in trouble, Boxer is the first one out there to defend the interests of the animals.  There is not a selfish bone in his body.  

If one were to make the case that Boxer might harm the other animals on the farm it would be in his lack of resistance.  Boxer does not object or speak out in an active manner about what is happening on the farm.  In continually saying that "Napoleon is always right" or "I must work harder," a case can be made that Boxer harms the other animals on the farm because he does not speak out.  Boxer does not use his superior strength to marshal the other animals in opposition to what is happening on the farm.  This argument suggests that Boxer could speak out in a more forceful voice of dissent and ensure that the other animals follow his lead.  I think that if Boxer is able to speak out more and ensure that other animals follow his lead, he could have helped the animals more in their predicament.  A stronger and more forceful Boxer can develop resistance against Napoleon and his brutality.  In continually ensuring that he needs to work harder and Napoleon is infallible, Boxer harms the other animals by not raising questions to the challenges within the system.  It is here in which one could argue that Boxer harms the other animals. Yet, this must be kept in context in that Boxer's tragic shortcomings are nowhere near as bad as what Napoleon does to the animals.