Mutated Bat

Modern scientists found out that the common bat has the inherent power of transformation. For this reason, bat blood is high-priced among alchemists and probably also explains the bats connection to vampires in some way. Due to certain influences by contaminated food or magical radiation, some bats transformed into horrible beasts. These beasts are well-known as Mutated Bats. Unlike intended mutations that origin from experiments, the mutation of bats seem to happen rather spontaneous.
Mutated Bats seem to lack the fertility of other mutated species as it is was never heard that they brought up offspring. The transformation into a giant-sized mutated bat seems to put some strain on the bats, turning them incredibly aggressive. Only the company of other Mutated Bats seems to calm them a little. Lonely Mutated Bats can become so aggressive that they attack their own shadow until they die of exhaustion.
In certain areas with a high magical contamination, the Mutated Bat has become a dangerous plague. Swarms of those bats have killed whole small villages in a single night. Moving only in the darkness, they devastate the land like a swarm of locusts until they are stopped by military or brave heroes.
As it is rather easy to create Mutated Bats on purpose, scholars of magic or alchemy often use them as watchdogs or tools of vengeance. Their aggressive and destructive nature makes such endeavours dangerous at best, and not few lost control of their vicious pet in a rather inconvenient moment.