The ending of Animal Farm is extremely pessimistic. The animals have traded a more lenient owner (Jones) for a brutal dictator (Napoleon) and their lives are harder than ever before. All the promises of a brighter future have been betrayed. They work as hard or harder for Napoleon than they did for Jones, and the pigs have become surrogate humans. The pigs now control the animals, and have mostly convinced them that they are being oppressed for their own good.
They were generally hungry, they slept on straw, they drank from the pool, they laboured in the fields; in winter they were troubled by the cold, and in summer by the flies.
The creatures outside looked from pig to man, and from man to pig, and from pig to man again; but already it was impossible to say which was which.
(Orwell, Animal Farm, msxnet.org)
The implication is that the pigs are now treated as equals by the humans, and since they have a unique insight into the mindset of other animals, they are able to exploit much more work for greater personal profit. Jones ran the farm normally, as a subsistence facility; the pigs run it as a slave plantation, where all the effort is done by the animals and all the benefits go to the pigs. The book does not continue past this point; both film versions showed the eventual collapse of the farm as its policies prove unsustainable.
The ending of Animal Farm gives a largely pessimistic view of the future, with only subtle hints of anything better. At the very end of the story, the human neighbours of Animal Farm come to visit and are entertained by the pigs. The other animals cautiously approach the windows of the farmhouse, driven by curiosity to see this first meeting of animals and humans on terms of absolute equality. They find that the human farmers and the pigs turn out to be in complete agreement as to the necessity of keeping down the "lower" elements in their societies and ensuring that the beings over which they rule do the most work for the least reward possible. Napoleon reassures the humans that the last traces of Animal Farm's revolutionary heritage are to be erased -- the insignia have been removed from the flag, for instance, and even the name of the farm is to be changed back to what it was originally, Manor Farm. Even though the scene ends in a violent quarrel when pigs and men discover each other cheating at cards, it seems that the net result of the animal revolution has been the consolidation of tyranny rather than its eradication. In these respects, the ending is quite pessimistic. The one development that might point towards the possibility of a brighter future is that the "lower" animals can now see clearly that the pigs are just as oppressive as the humans. The illusion of the solidarity of animals against humans has been shattered, and the true nature of the pigs' rule has become plain. This in itself will not result in any change for the better, but it is a necessary first step before change can become possible.