Killer Bat is a fictional comic book supervillain and a strong adversary of Frankenstein's. This was a bat that was mutated by Dr. Magnus Mysterious. Once mutated, it grew in size, strength and speed. It was the same strength as Frankenstein but was way faster than he was. Eventually, he becomes the pet of a kid named Kid Jimmy (Jimmy Lobber in the comics).
Killer Bat debuts in Season 2, Episode 4, "The Killer Bat, Part 1." There, Dr. Zing was observing Dr. Magnus Mysterious complete his latest experiment. Dr. Magnus Mysterious used a bat as a product tester in something called a DNA Chemical Genetics Tube (DNACGT). After being mutated, everything about the bat grows proportionally at about 15x its regular size. Dr Magnus Mysterious then creates Killer Bat and it flies out the window, searching for human blood. After stealing a lot of blood, it runs into Frankenstein. Frankenstein prepares to stop it but Killer Bat immediately attacks him and starts thrashing him left and right. Frankenstein then throws it to a trash can and goes up to a rooftop. There, Killer Bat follows from under him and then pops out and smashes down on him. With Frankenstein defeated, Killer Bat prepares to take his blood.
Next in "The Killer Bat, Part 2," Frankenstein throws some dirt at Killer Bat and jumps down the building and lands on a car. Killer Bat then searches for him but is nowhere to be found. Killer Bat goes back to Dr. Magnus Mysterious and gets healed from the scathe he got during his fight with Frankenstein. When going out to get more blood, Frankenstein stops him from taking it from a mere dog. Frankenstein then rams Killer Bat into the Empire Nations Building. Killer Bat comes out and scrapes Frankenstein on the chest. Frankenstein gets up and he and Killer Bat are fighting in air until Killer Bat throws him to the Nightmare City Carnival. There, they are in the mirror room and Frankenstein uses this to his advantage to get Killer Bat confsued and manages to beat down on him, thus defeating him.
Killer Bat returns in Season 4, Episode 3, "Killer Animals." He is spotted by many to be back with sucking blood. Eventually, Percy and Jen finds him and they are running as it is about to suck their bloods. When Killer Bat gets them, Frankenstein kicks him but then Killer Wolf and the Wilderbeast comes in as reinforcements. The three were then able to overwhelm Frankenstein and co. When Frankenstein wakes up, he finds the three animals preparing a feast. He then realized that they were going to eat them. Frankenstein manages to bust both him and Jen out and tells Percy to distract them. As Percy distracts them, Killer Bat screams in his face and the trio chases him to the roof where he is about to fall. When he does, Killer Bat races to get him but JoeDanny gets to him first. Killer Bat is then defeated when Frankenstein throws a giant rock at the door and flings him to the dumpster.