Most of the classic monsters are not actually separate species but humans that have been damaged or changed somehow.
- Vampires are just humans that have been infected with a virus
- Werewolves are also humans that have been infected with a virus
- Zombies are likewise virus-infected humans
- Mummies are dead humans that have somehow come back (usually through magic)
- Skeletons are the same as zombies/mummies, but without their flesh
- Witches are humans who use magic (usually for evil)
- Ghosts are dead humans
Even more contemporary monsters like Jason Voorhees, Freddy Kruger, Michael Myers, Pinhead, Chucky, creepy Japanese girls, etc. are just humans that have been changed in some manner.
Of course, there are some monsters that are not human whatsoever, for example the Creature from the Black Lagoon, Godzilla, demons, and such.
It is interesting however to realize that most monsters tend to be human. If one were to analyse that fact from a psychological viewpoint some interesting/obvious reasons would likely be found.