Gwythaints had talons as sharp as daggers, beaks like axe-blades, and black feathers. Their attack method was to swoop down on enemies, like real raptors.
This young creature, found and nursed back to health by Taran during the events of The Book of Three (novel), later appeared in The High King, and proved crucial to the overall plot and themes of The Chronicles of Prydain. See the above link for details.
The gwythaints first appear in the movie as they kidnap Hen Wen from Taran, and appear a second time when they are fed food by one of the men. They also attempt to chase Taran, only to end up being chained to their perches. Later the Horned King sends them out to follow Taran and his companions. The gwythaints manage to find them and the Black Cauldron. After the Horned King meets his demise, one of the gwythaints is killed while another escapes with Creeper.