Birdzilla was the name that Chance gave to a turkey that was on Kate's farm after he encountered it at the chicken coop. The turkey intimidated him, and forced him to flee in terror. Birdzilla was also what led to Chance to change his mind after chosing to stay regarding escaping Kate's farm with Shadow and Sassy to return to the Seavers' home. Birdzilla was not seen afterwards, probably been returned to the chicken coop by Kate's friend Frank.
Going by its actions during its appearances (including attempting to pursue Chance while he attempted to escape), Birdzilla is extremely territorial.
Birdzilla is a portemantau of "Bird" and "Godzilla."
Ironically, considering Birdzilla's species, Chance is shown to love turkey in the beginning and ending of the film.