There is a scene where the Reverend Mother Gaius Helen Mohiam meets secretly with Barron Harkonnen and his mentat, Piter de Vries, to demand that Jessica and Paul be left unharmed. At the start of the scene there is a large black "spider" referred to as "our pet". Mohiam believes it is sentient, saying "It understands," in response to Piter's assertion to the contrary.
What is this creature? Is it a Guild navigator?