If I had to divide classic monsters into four branches, it'd probably be something like
Demon - obvious here. Maybe branches into succubus/incubus, imp/gremlin, and "outsider" tentacle thing.
(Physical) Undead - branches into mummy, zombie, frankenstein (explicitly the monster's name in this canon), vampire.
Some sort of, I dunno, primordial spirit? It's a homogeneous thing that branches into other homogeneous monsters, like proper ghost, slime, swamp thing, and golem or w/e.
Finally, the most hideous, terrifying form of them all: Man. Branches into werewolf, and various forms of "modified man" like Jekyll/Hyde, Invisible Man, slasher villain, and maybe reality warper? I could swear there's stuff that included it beyond the twilight zone's cornfield kid.