On one Thanksgiving holiday, Mr. Fairless purchased a box of Happy Turkey brand stuffing. Fairless was completely unaware that the stuffing was haunted. After the turkey was stuffed with it, the entity possessed the turkey through exothermic ectoplasmic metastasis while it cooked in Fairless' oven. It forced the bird to grow in size and transmogrify to sprout feathers, even. Based the ability to mimic animal behavior, it appears the ghost was much "smarter" and coherent than a standard possessor entity.
To further complicate matters, the turkey acted as a Protective Psychokinetic Barrier, a veritable suit of armor, and nullified the effect of Proton Streams. Since the stuffing was where the spirit was, the Ghostbusters faced a difficult task of removing it from the turkey.
The Possessed Turkey has the standard power to levitate. Aside from possession, it can also animate objects in the immediate vicinity.