Are They Rhinos?

[The Saucepan Man]

they are an invention of Peter Jackson & Co

Not entirely, Son of Númenor. As Elrond of Rivendell said in the thread to which Thorongil linked, Tolkien does refer to great beasts pulling Grond:

Grond they named it, in memory of the Hammer of the Underworld of old. Great beasts drew it, orcs surrounded it, and behind walked mountain-trolls to wield it.

I don't think that Tolkien ever describes the beasts, so their appearance is pretty much a matter of interpretation. Personally, I think the designers did pretty well. They appear to be based on a prehistoric mammal called an Arsinoitherium which, despite its appearance is not related to the rhinoceros, but belongs to an extinct group.

Edit: Ah, found some pictures of the beasties