The abuse of power is foreshadowed several times in Animal Farm.
First of all, you've got Napoleon. The name in itself is foreshadowing. Napoleon, the man, was an ignoramus who wanted nothing more than power. He didn't really care about the people he stepped all over to get it, so long as he got it. Napoleon, the pig, is eventually brandished as having a personality all too similar to that of the man.
Then there's the AWOL milk incident. It's obvious that the pigs did something with it:
"'Never mind the milk, comrades!' cried Napoleon, placing himself in front of the buckets. 'That will be attended to. The harvest is more important. Comrade Snowball will lead the way. I shall follow in a few minutes. Forward, comrades! The hay is waiting.'
So the animals trooped down to the hayfield to begin the harvest, and when they came back in the evening it was noticed that the milk had dissapeared."
Napoleon Crossing the Alps
Napoleon's Return from Elba, by Charles Auguste Gu...
Napoleon on his Imperial throne--page 44, last two paragraphs
The animals later discover that the pigs, indeed, had done something with the milk. They had decided to mix it into mash, and to also eat all of the apples. The animals, who should have been outraged, weren't, only thanks to the eloquence and quick-thinking of Squealer:
"Comrades! You do not imagine, I hope, that we pigs are doing this in a spirit of selfishness and privilege? Many of us actually dislike milk and apples. I dislike them myself. Our sole object in taking these things is to preserve our health. Milk and apples (this has been proved by Science, comrades) contain substances absolutely necessary to the well-being of a pig. We pigs are brainworkers. The whole management and organisation of this farm depend on us. Day and night we are...