Sub-Sider

The Sub-Sider is the devolved form of starving Vampires.

Apparently, vampires need human blood to stabilize their DNA, so if vampires don't get enough human blood, they turn into these hideous & feral bat-like creatures, and once this process has taken place, not even human blood can reverse it. These creatures live in the sewers, coming out to find blood every time they can and most are so desperate that they will go into people's homes to find food. One ventured into Edward Dalton's apartment, but he and his brother manage to kill the creature, taking off its head with a knife, while it swung upside down from the ceiling.

Also, whether or not fully formed subsiders can fly is unknown, but if they can, it could explain how the subsider got into Edward's apartment in the first place.