In "The Slave's Dream" by Longfellow, what are the elements which show that the slave belonged to Africa?

I am not sure that "belonged" is the right word. Perhaps
"came from" would be better, but either way, you need to look at the central dream
sequence that the slave experiences during his death-slumber. The one detail that
definitely links it to Africa is reference to the river Niger, which is mentioned twice,
first as he overlooks the scene which includes "the lordly Niger" and secondly as he
rides his horse along the banks of the Niger. In addition, reference to the "bright
flamingoes," the "tamarind," and other such animals as the lion and the hyena all
clearly link the slave's dream to his home country back in Africa. Examining such
details clearly indicates that this is a very strong and powerful dream of the liberty
that the slave formerly enjoyed, which of course only serves to highlight the horror of
his present state, being taken so far away from his home and forced to work under
terrible conditions.