What is the significance of the images of the flamingos and the river-horse in the poem "The Slave's Dream"?

Dreams are manifestations of man’s deep-seated desires, fears and other emotions. The yearning for liberty is the strongest and deepest feeling of a slave. Finding it impossible to be realized in reality, he takes refuge to dreams. Though unreal and illusory, they offer him a platform to fulfill his desires.

Whatever the slave sees in the dream reflects his pining for freedom. Flamingos and river-horse are among many other images that he associates with the idea of liberty. The poet says,

Before him, like a blood-red flag,
  The bright flamingoes flew;
From morn till night he followed their flight,
  O'er plains where the tamarind grew,

He sees “bright flamingoes” fly “from morn till night.” Riding on his horse, he follows them throughout the day till he reaches the ocean shore.

A flying bird is a stereotypical image of freedom. Thus, flamingos, in the poem, symbolize liberty. Like them, the slave wants to move unrestrictedly; though in reality his movement is restricted and confined.

In the next stanza, the poet says,

At night he heard the lion roar,
  And the hyena scream,
And the river-horse, as he crushed the reeds
  Beside some hidden stream;

River-horse is another name for hippopotamus, a mammal indigenous to Africa. They are semi-aquatic creatures that mostly inhabit water bodies including rivers, lakes and mangrove swamps. They spend the day mostly in water or mud, and come out for grazing grasses after sunset.

In his dream, the slave hears the movement of the river-horse “beside some hidden stream” “as he crushed the reeds.” Unlike the slave, a river-horse is independent and self-determining. It moves freely in water and on land.

What’s common between the flamingos and the river-horse is that both of them move unrestrictedly and freely. The flamingos have the whole sky as their abode, while the river-horse moves at liberty in water and on land.

So, we see that the images of flamingos and water horse contribute to the thematic structure of the poem, and underscore the slave’s desperate craving for freedom.