#mm!

Scientist #1:

#1 in a pair of not too bright scientists, who in the name of Dagget is "always trying to hurt us in the name of science". He has no canon name, which is a running gag to the point that when he gets a focus episode "Scientist #1" is in the episodes title.

Dr Stylish:

Possibly the gayest mad scientist to not appear in Rocky Horror, I honestly have no idea what this guy's first name is, or if Stylish is even his real last name, he's only ever referred to as "Dr. Stylish"

Buzz Kutt:

He appears to be a well respected scientist in the monster world, but his line about "conquering the humans" something no other monster seems interested in, seems to imply he's a mad scientist, even by their standards.

Noa Hiiragi:

Only in high school, but has an IQ over 200, and has a plan that involves altering the minds of everyone on the planet.

Dr Finklestein:

Literally the living embodiment of the concept of "Mad Scientist" in Halloween Town. I'm actually not sure if he has a Doctorate or not, but he's called "Dr. Finklestein" so Maybe?

Dr Werner Frank:

Inventor of FRANXX mechas the series is named after…. and also cloned and experimented on poor Zero Two when she was a child.

Vink:

He calls himself "Dr. Vink", but I am skeptical he is a licensed Doctor. Also insists he is not a "nutbag"

Fran Madaraki