It goes without saying that bats, of all shapes and sizes, are the most common creature found in the various games in the Castlevania series. The classic monsters of horror all make their appearances as bosses, but when it comes to populating the halls of the castle with creatures, nothing does the trick like a bat. It feels proper to have bats around as Dracula is a vampire, and vampires can turn into bats. It's like a transitive property, then, to put bats in the castle of a vampire. That just makes sense.
But while bats, and vampires, are regular fixtures in the series, rarer are another kind of bat-like being, the man-bat. This is a human who has, through magical or scientific means, been fused with a bat to create a hybrid creature. We didn't get a proper man-bat until Castlevania: Legends in 1997. And then, after, not really again. A hulking, bat-like beast to patrol the halls of Castle Dracula should be a fixture, you'd think, but it isn't.
That makes sense, though, when you think about it since the Giant Bat was introduced in the original Castlevania and it served the same purpose: a giant, hulking, bat creature. In fact in Legends the Creatures Bat is functionally similar, patrolling the first zone of the game as a Giant Bat would. Thus, this game just added in a stylistically different but functionally similar creature to give the boss some flair.
As the reigning Belmont of her era, heroine Sonia Belmont had little choice but to head to Castle Dracula when the vampire lord threatened the populace of Wallachia. Although young and inexperienced in vampire hunting, she knew she was the only one that could battle the vampires and other creatures of the night, as her family had been doing since the days of Leon Belmont. To do any less would be ignoring her sacred responsibility.
So she trudged to the castle, battling her way through the graveyard that marked the exterior grounds of the haunted fortress. She worked her way through man traps and pit falls in this unholy ground all before facing off against a horrible beast, the Creatures Bat. This fiend, the fusion of bat and man, attacked roughly and quickly, leaving her little time to react. But despite the year of experience she was due, Sonia still rose to the challenge, striking out with her family's sacred whip, fending off the beast. In the end she came away victorious, in this her first of many battles in Castle Dracula, and the fight against the Creatures Bat gave her the confidence to go onward, deeper into the halls of evil.
Other Versions of Creature Bats:
While the official Creatures Bat is limited only to Castlevania: Legends, other half-human half-bat creatures have appeared in various games. The most prominent were the succubi (all all their various forms) in Castlevania: Circle of the Moon. In fact, most versions of the Succubus could be considered a kind of Creatures Bat as she is normally depicted with bat-like wings on her human body.
By a similar measure, many of the demonic "last form" versions of Dracula are also bat-like in their composition. Fusing humans with bats in a motif the Castlevania designers clearly love.
And, if we extend the concept out just a little further, the Death Bat from The Castlevania Adventure is another weird bat-beast hybrid. This one more closely resembles a bird and bat hybrid, in skeletal form, making for its own horrific visage.
Fighting Against Creatures Bat:
The fight begins with the Creatures Bat swooping in from the top of the screen. He will then proceed through a very simple pattern. First he will lower himself close to the ground, flapping his wings. This is a good time to jump and attack, and with good timing you can get three to four shots in. Then the bat will fly horizontally across the screen and you should duck to avoid this. After this attack he will then fly up to the top of the screen and flap his wings quickly, over and over, before lunging down to wherever Sonia was just standing. You have to move quick to avoid this. Once he's done both of these attacks, though, he'll repeat the process, over and over. You should be able to easily learn his pattern and avoid his attacks, taking him out quickly.
Creatures Bat in Pop-Culture
One can find various fusions of animals and humans throughout mythology and pop-culture. When it comes to bat-men, though, the most prominent examples really come from comics. DC has the Man-Bat, an obvious villains parallel to Batman, who was a human who fused with bat extract, creating this villainous creature that stalked the night. By a similar coincidence (in that it wasn't coincidence at all as comic publishers regularly stole from each other), Marvel had their own man-bat hybrid, Morbius, who fused his own body with that of vampire bat genetics, creating his vampire-like form.
Of course, if you want to skip the comics and just watch a film, Sony did make the Morbius film (but, really, you're probably better off reading the comics as that movie is terrible).