You asked two questions and so, according to enotes regulations, I have had to edit it down to one question. Boxer is a very important character in Orwell's allegory about the tyranny of power. From his very first introduction, it is clear that Boxer is renowned for his strength. He is large, standing at about six feet tall, and is characterised by his unfailing devotion to the idea of Animal Farm. Again and again, his incredible strength is of vital assistance to the farm's development, protection and survival, and Boxer gladly throws himself into his work for the good of the farm. He is likewise an important military asset, proving a key soldier in the defence of the farm when it is attacked twice. His personal slogan emphasises his self-sacrificing nature: "I will work harder."
However, it is clear that the strength of Boxer is at least partly balanced by his lack of intelligence and his gullibility. Orwell mentions that he was only able to learn the alphabet up until the letter D, and he is only able to think and remember simple slogans, which make him particularly susceptible to the manipulative rhetoric of Napoleon. Note how easily Boxer is persuaded that Snowball was a traitor:
"Our Leader, Comrade Napoleon," announced Squealer, speaking very slowly an firmly, "has stated categorically - categorically, comrade - that Snowball was Jones's agent from the very beginning - yes, and from long before the Rebellion was ever thought of."
"Ah, that is different!" said Boxer. "If Comrade Napoleon says it, it must be right."
Even, towards the end of his noble life, when he collapses whilst building the windmill, his thoughts immediately go to the work rather than his own health. Note how his death is another example of the despotism of Napoleon - he thinks he is being taken to his retirement and needs to be told the truth by Clover and Benjamin, but too late.
Boxer is seen as a threat to Napoleon for a variety of reasons. Boxer is very strong on both physical and political levels. Napoleon, with the help of Squealer, comes to view Boxer as a threat because of this and something that has to be controlled. At the same time, Boxer is a very authentic and transparent figure. His questioning, unlike Benjamin's, is not necessarily done out of a cynical point of view, but rather out of a genuine desire to understand and to know. Boxer's function is as a workhorse, but is also one towards whom others gravitate. He is well- liked by others because of this loyalty and transparency. It is for this reason that Napoleon and Squealer see him as a potential threat to be neutralized. While the revolution of the animals might have been believed by Boxer, in the end, his presence is seen as a threat to the consolidating and coalescing power of Napoleon. When Napoleon cuts the deal to get rid of him, it is done for political expedience for once the greatly heroic figure of Boxer is gone, there will be little political source for dissent, ensuring his power becoming even more unchecked.
Boxer symbolises the gullible, deluded, naive, innocent and faithful source of labour, the fundamental workforce dedicating themselves to their leaders who came to the power treacherously, who exploited and abused the good will of blinded supporters, the victims of Napoleon and his aides who misinterpreted the new philosophy and corrupted absolutely when they had absolute power, uncontrolled unquestioned, intimidating ruling body, whose benefits came before the interests of the Animal Farm, who distorted the facts to their own advantage, who betrayed their labourers. Napoleon, enslaved by his ambition to be a dictator, despotic leader, tyrannised his own low level class, working body, who played a great role in building Animal farm free of man. Boxer's sacrifice and devotion is not awarded but his power and admiration for his strength by other animals forms a threat to other animals' uprising. So his traces are erased by his inevitable death planned by his malicious leader, Napoleon. Boxer-like people are implied to deserve defeat due to their too excessive loyalty. The writer is trying to warn Boxer like people to raise their awareness and not to be blind to reality, that their lives will be miserable and they will suffer and pay a high price if they continue to support tyrants with too much devotion. They will be used and thrown away when their existence jeopardises the system established to terrorize nations with anti democratic regulations.