Beann'shie is one of Deidre's black wolves, who would not leave her side, even to hunt.
If Geralt refuses to deliver Deidre into Sabrina Glevissig's hands, but then tells Deidre to lay down her sword, D'yaebl and Beann'shie aid their mistress when she engages the witcher in a fight.
Significant plot details end here.
Beann'shie is a word for banshee. (Elder Speech)