The animals' have more than one enemy. As the novel opens the animals' enemy is Mr. Jones and animals' enemies everywhere are generally the farmers and humans they feel oppress them. As the novel progresses and the animals do away with their human enemy the enemy becomes Snowball. Napoleon uses Snowball as his scapegoat after he runs him off the farm. The animals grow to hate Snowball and to blame the farm's misfortunes on him. The farm cannot sustain the rule of Napoleon for long and soon the animals begin to see him and the rest of the pigs as the enemies because they use all the farm's resources without spreading the wealth evenly.
This comes from Old Major's first speech:
"...Why then do we continue in this miserable condition? Because nearly the whole of the produce of our labour is stolen from us by human beings. There, comrades, is the answer to all our problems. It is summed up in a single word - Man. Man is the only real enemy we have. Remove Man from the scene, and the root cause of hunger and overwork is abolishyeed for ever..."
The animals' have more than one enemy, though some aren't revealed until later in the story. Their most obvious enemy is 'Man', due to the abuse suffered. This hatred of man and fear of Jones is what keeps them from seeing who their real enemies are. Later on, its apparant that their real enemies were among them all along, perverted by their luxuries and thirst for power. Napoleon and Squealer, as well as the dogs, have become their enemy, and have taken on the same role that Jones had before he was ousted. They've even started an institution of tyranny through newly arriving pigs. You also have Pilkington & Fredericks, two farmers who try to find ways to use and trick the animals through trading with them, only to have Fredericks open attack them. One could also say that their most dangerous enemies are themselves; for whenever they notice something is wrong or unjust, its always that interior argument 'Napoleon is always right' which keeps them from doing anything about it.