Pumpkins seemed to be a suitable substitute for the turnips and pumpkins have been an essential part of Halloween celebrations ever since. Pumpkins were cut with faces representing demons and was originally intended to frighten away evil spirits. It was said that if a demon or such were to encounter something as fiendish looking as themselves that they'd run away in terror,thus sparing the houses dwellers from the ravages of dark entities. They would have been carried around the village boundaries or left outside the home to burn through the night. Bats, owls and other nocturanal animals, also popular symbols of Halloween, were originally feared because people believed that these creatures could communicate with the spirits of the dead.
Black cats has religious origins as well.
During the Middle Ages it was believed that witches could turn themselves into black cats. Thus when such a cat was seen, it was considered to be a witch in disguise. Witches and witchcraft are dominant themes of the holiday. Witches generally believe themselves to be followers of an ancient religion, which goes back far beyond Christianity, and which is properly called 'wicca'. Let no one be found among you who sacrifices his son or daughter in the fire, who practices divination or sorcery, interprets omens, engages in witchcraft, or casts spells, or who is a medium or spiritist or who consults the dead.