The word that is translated as unicorn was a symbol of Manasseh, a tribe that lived near Mt Hermon ("Sirion"). No one's really sure what the animal actually was. Rhinoceros is a common choice, because it can also be used as symbolism of the unequal importance of Joseph's sons (Manasseh and Ephraim), because it has a large horn and a small horn. And really, a mountain running like a rhinoceros is a great metaphor.