Looks like one of Jimmy Neutron's incomplete inventions A.K.A a Stub.
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[Note: In this episode, the usual atom transitions are replaced with Jack O' Lanterns, to reflect with the Halloween theme.]

Vampire Carl: I want more! I need blood!

Sheen: You need counciling.

Vampire Carl: Must have blood! Listen to them, the children of the night. What music they make. I must join them! Bye you guys! See you later!

Sheen: That was weird. But totally awesome! I wanna turn into a bat! I wanna fly! Make me a bat, Jimmy! Make me a bat!

Jimmy: I didn't do that! Carl just changed into a vampire bat on his own.

Sheen: But it's Halloween. What do you expect?

Jimmy: The monster maker must've mutated Carl's molecular structure on a subatomic level, altering his DNA!

Sheen: Hmm? And now again in English.

Jimmy: He's a real vampire!

Sheen: Cool!

Jimmy: Not cool! If Carl turned into a real vampire then you could turn into a real...! (gasps)

Werewolf Sheen: (Snarling and Growling as the clouds uncover the full moon)

Jimmy: Uh Sheen, I think we better go back to the lab.

Werewolf Sheen: (Growls then attempts to attack Jimmy)

Jimmy: Or not. This is going to be a memorable Halloween Goddard... if we survive.