All monsters belong to a monster class, based on the default ASCII monster symbol that represents them. For some classes, the symbol is almost all that unites them; for instance, x or w. Other symbols represent monsters that share much in common, such as D or K. Additionally, some classes have individual members that are in many ways an "odd one out", such as E stalkers and e gas spores.

Current classes are as follows. Names are as provided by the NetHack what is command.

• a ant or other insect
• b blob
• c cockatrice
• d dog or other canine
• e eye or sphere
• f cat or other feline
• g gremlin or gargoyle
• h humanoid
• i imp or minor demon
• j jelly
• k kobold
• l leprechaun
• m mimic
• n nymph
• o orc
• p piercer
• q quadruped
• r rodent
• s arachnid or centipede
• t trapper or lurker above
• u unicorn or horse
• v vortex
• w worm
• x xan or other mythical/fantastic insect
• y light
• z zruty
• A angelic being
• B bat or bird
• C centaur
• D dragon
• E elemental
• F fungus or mold
• G gnome
• H giant humanoid
• J jabberwock
• K Keystone Kop
• L lich
• M mummy
• N naga
• O ogre
• P pudding or ooze
• Q quantum mechanic
• R rust monster or disenchanter
• S snake
• T troll
• U umber hulk
• V vampire
• W wraith
• X xorn
• Y apelike creature
• Z zombie
• @ human or elf
•   ghost
• ' golem
• & major demon
• ; sea monster
• : lizard