Nien Nunb and Admiral Ackbar are symbols of the 'secular' nature of the Rebel Alliance. The Rebel Alliance had a number of non-human members; even at high positions. On the other hand, the Galactic Empire had it's marked biases against non-human species. Non-humans were either prisoners or slaves, in their scheme of things.
Nien Nunb was the co-pilot to Lando Calrissian on the Millenium Falcon at the Battle over the Endor moon to destroy the second Death Star.