Cliff-ghasts are vulture-like flying creatures that live in the North. Cliff-ghasts can communicate in a very primitive kind of language.
Since many of the characters grow up in the world they come from, some having encountered them before the start of the Northern_Lights they know what these creatures are and consequently no character at any point explains them, so we can only guess.
They can fly, and are mortal. Lee Scoresby, who hates killing sentient creatures, has no apparent qualms about killing these monsters. They are scavengers and enjoy killing. In the play adaptation they are depicted as hooded and shrouded.
Cliff ghasts can speak, though they do not converse with any of the characters. They are heard twice: once overheard and once talking amongst themselves as they butcher an arctic fox. They are at first apparently one of the magic elements unique to the world of the Northern Lights, but then appear in other worlds. When the great war begins, they are the only being we know of not to take sides, merely waiting to feast on the casualties. They are certainly repugnant, though not necessarily evil. They live in groups.