Size: A wingspan of 10 feet.
Appearance: Typical of a bat, only much larger. Wide, leathery wings, a body covered in grey fur, and large, dark eyes. Some reports describe the face as almost monkey-like.
Threat: Low. It has been known to swoop down at people in boats, but reports of it outright attacking and inflicting injuries are rare. The aggressive behavior it takes fits in with similar actions by animals seeking only to scare away intruders into their territory.
Fortunately, for those who still have nightmares based on The Wizard of OZ, despite its ability to fly, the Ahool remains reported exclusively on the island of Java in the Philippines. Although reports of other large, bat-like creatures have also been reported in Pakistan, Afghanistan, and in the jungles of India.
It's unsurprising that bats are included in the animals of which we fear growing to giant size. Even as small as most species are, they tend to send people running as they flitter about our heads at night. Their leathery wings and unusual way of locating prey, as well as their nocturnal habits all feature into things which confuse and frightened primitive man. The way some species subsist on the blood of much larger animals also brings a certain level of fear and it part of the reason they were connected with vampires. As we've learned more about them, we've even found new reasons to fear them, with the addition of Rabies and other similar diseases they can carry.
The Ahool is one of those creatures, though, that actually has a fair explanation.While there isn't any direct evidence of the creature the Ahool is claimed to be, there is a similar animal which is very rare that lives in the jungles of Java. The Javan Wood Owl is a good candidate to be the creature. Its wingspan is a far cry from that reported of the giant bat, only reaching an average of four feet, but witness estimates of such things are easily exaggerated, particularly at night and in a panic at having something swoop over your head. As the owls have been known to do to people hiking and working near their territory.
Still, the idea of giant bats has the ability to haunt human dreams and the invariably pop up in several movies, TV shows, and books. There is a reason we have Batman and not Nightman, after all. There is just something unnerving about them which bites into us.
As far as entertainment goes, there is The Devil Bat starring Bela Legosi, and The Bat with Vincent Price. More recently are the movies Bats and Vampire Bats (Which I don't believe are related, though I must admit to not having seen them). There is also Attack of the Bat Monsters and The Bat People for those who enjoy the older now-campy monster movies.
So, there really isn't much to be afraid of honestly, as far as bats go. Some may carry Rabies, but those cases are getting rarer and rarer as animals that are even suspected are trapped and destroyed. They're not really out to eat us, or suck us dry. Usually.