Piotyr is Gregor's dog who was killed a few days before the events of the story. Currently residing in Bina's head, she emerges in Bina's dreams, or when Bina is in the Moment. Piotyr's body is a large monster, known as the Hound, reanimated by the Botfly which protects Gregor.
Piotyr is a beagle. She belonged to Gregor. She liked to play running-away games. She liked to sniff at things. The combination of these two traits led to her being crushed by a piece of construction equipment. Gregor was desperate for her to be rescued somehow, so the Botfly split her into a ghost and a physical form; the ghost stayed with Bina, and the physical form became the Hound.