Ever wonder why bats are considered so spooky and closely tied to Halloween images? There is not much documentation on this subject. However it is most likely a combination of many factors that contributed to bats being a Halloween symbol.
In ancient times the festival of Samhain was celebrated in Ireland. This end of the harvest celebration is the origins of our modern Halloween. Along with dressing up as spooky characters (to scare away or blend in with the spirits that were believed to return to earth) another tradition was bonfires. It is believed that the fires attracted insects to the area which in turn attracted bats. So over time bats became associated with Samhain and our current Halloween celebrations.
The vary nature of bats can be quite eerie and spooky. They live in dark caves, sleeping upside down, only coming out at night. When they leave the caves at twilight they do so in great numbers. It is an awesome sight to see hundreds, even thousand of bats flying into the night. But it can be a scary sight when so many are gathered in one area. They are known to "swoop" down out you. In reality they are not after you (or your hair, a common misconception) but any bugs that may be flying in the air around you.
They are carriers of the rabies disease. So there is a legitimate fear of being touched or bitten by a bat. This was especially true years ago prior to rabies vaccinations for pets and current shots to prevent the disease in anyone who is bitten. Untreated rabies is almost 100% fatal and is not a pleasant way to die.
With the birth of Dracula in books then eventually movies bats became even more famous. These original vampires were depicted with the ability to transform into bats. Why bats? Most likely due to the above reasons plus the real bat species now called vampire bats which do feed on the blood of animals.