Some entries can also be interpreted as alternate names for existing monsters.
Volume I:
daughter of the tempest = storm giant
face of rays = beholder
flesh dissolver = digester
grimling = meenlock
grotesque = gargoyle
monster man = mongrelman
murk beast = shadow mastiff
pigman = orc
psychic ambusher = intellect devourer
risen dead = any undead
scourge willow = black willow
sea witch = sea hag
spectral follower = invisible stalker
tentacle terror = roper
tocsin bloom = shrieker
walking filth = shambling mound
web lord = aranea/ettercap
Volume II:
animalcule = minimal
brambleman = needleman
cragman = galeb duhr/vilstrak
embrangler = umber hulk
enantiomorph = what a mirror of opposition summons
fatal floor = trapper
fitchew = giant weasel
flammivomous = any fire-breather
formless fury = chaos beast/gibbering mouther
gabble ratch = aarakocra/kenku
hastilokest = quickling
hawkhead = hook horror
hellbred = cambion
holy roller = juggernaut
king of the cats = cat lord
life-in-death = any undead
lumbrike = purple worm
maid of the oaks = dryad
mortmain = creeping claw
mother of mischief = deepspawn
nipperkin = crabman
rogalian = any fire creature
scary silhouette = shadow
viscous killer = oozes/puddings