Identifying climaxes in a story can be a tricky business because it is sometimes a matter of opinion. In a book length story like Animal Farm, there are, in addition to a major climax near the end of the story, frequently one or more minor climaxes that bring subplots or minor aspects of the storyline to a close.
I believe that the major climax to Animal Farm occurs when we see that the pigs have completely dropped the pretense that they are trying to avoid the dangers of acting like humans. They change the name of the farm back to its original name (Manor Farm), they begin trading with humans, and they begin walking on two legs. Finally, the animals can no longer tell the difference between the pigs and the humans.
One minor climax occurs when Boxer finally collapses and is carried off to the horse slaughterer. This brings to an end the plot line in which Boxer figured prominently as a loyal, hardworking “citizen” who is taken advantage of and finally worked to death.