Monsters are creatures of legend. Some have powers. Some more unique or exotic than others. They include creatures such as mummies, vampires, werebeasts, and many more. Humans are also in the category of monsters somewhere, but not really. They are in the category of a species.
Monster High, being a monster-themed franchise, features a huge cast consisting of many different types of monsters. And some humans to round it out, of which most fit the conventions of the horror genre. Below is the entirety of the cast listed by species, with those who are either unknown or unique to the franchise listed as "other".
Please keep two things in mind when reading:
The monster history articles linked to are not necessarily fit for reading by a young audience.
Characters are primarily grouped the way canon does it, even if canon would conflict with source conventions. Where canon is silent, source conventions are utilized.