One of the things that makes NetHack totally Awesomely Powerful and World-Famous is that it uses ASCII characters for graphics. In particular, each creature is represented by a letter or a punctuation mark -- these are called "classes". The organization of NetHack creatures into classes is loose, but it's definitely there. Like 'q' probably indicates 'quadruped', and 'r' indicates rodent.

At any rate, this does have some game ramifications. In particular, a blessed scroll of genocide allows you to eradicate a "class" of monsters; that is, all creatures that are normally represented by a given symbol. So it is useful to have a good idea of which creatures are which letter. Furthermore, just because 'q' represents rothes, which are easy to beat, don't go rushing up to every 'q' you run into (especially if you don't play in color) -- 'q' is also used for mumak.

NetHack classes are as follows:

And if we could use tables I wouldn't have to write this pesky definition list

A
Angels
Angel
Archon
couatl
Aleax
ki-rin
B
Bats
bat
giant bat
vampire bat
raven
C
Centaurs
plains centaur
forest centaur
mountain centaur
D
Dragons -- all colors
E
Elementals
earth elemental
air elemental
fire elemental
water elemental
stalker
F
Fungi
lichen
brown mold
yellow mold
green mold
red mold
shrieker
violet fungus
G
Gnomes
gnome
gnome lord
gnome wizard
gnome king
H
Giants
stone giant
hill giant
fire giant
frost giant
storm giant
ettin
titan
minotaur
I
Something that is invisible or cloaked by darkness
J
Jabberwock
K
Keystone Kops
L
Liches
lich
arch-lich
master lich
[lich

M
Mummies
N
Nagas -- both hatchlings and full-grown
O
Ogres
ogre
ogre lord
ogre king
P
Slimy, very dangerous things
gray ooze
black pudding
green slime
brown pudding
Q
Quantum mechanics
R
Will destroy your stuff
rust monster
disenchanter
S
Snakes
garter snake
pit viper
snake
water mocassin
python
cobra
T
Trolls (all)
Olog-hai
U
umber hulk
V
Vampires
vampire
vampire lord
Vlad the Impaler
W
Wraiths Etc
wraith
barrow wight
Nazgul
X
xorn
Y
Primates
monkey
ape
owlbear
yeti
sasquatch
carnivorous ape
Z
Zombies
a
Insects
giant ant
soldier ant
fire ant
giant beetle
killer bee
queen bee
b
Blobs
acid blob
quivering blob
gelatinous cube
c
Cockatrices
chickatrice
cockatrice
pyrolisk
d
Canines
jackal
coyote
fox
little dog
dog
large dog
werewolf
werejackal
winter wolf cub
winter wolf
dingo
warg
hell hound pup
hell hound
e
Floating Spheres
floating eye
gas spore
freezing sphere
flaming sphere
shocking sphere
f
Felines
kitten
housecat
panther
jaguar
tiger
lynx
large cat
g
Gremlinoids
gremlin
gargoyle
winged gargoyle
h
No idea
hobbit
dwarf
dwarf lord
dwarf king
bugbear
mind flayer
master mind flayer
i
Impish
imp
quasit
lemure
homunculus
manes
tengu
j
Jellies
blue jelly
spotted jelly
ochre jelly
k
Kobolds
l
leprechaun
m
Mimics
n
Nymphs
water nymph
wood nymph
mountain nymph
o
Goblinoids
goblin
hobgoblin
orc
Mordor orc
hill orc
Uruk-hai
orc shaman
orc-captain
p
piercers
q
Quadrupeds
rothe
mumak
leucrotta
titanothere
baluchitherium
wumpus
r
Rodentia
sewer rat
giant rat
rabid rat
rock mole
wererat
woodchuck
s
Arachnids
cave spider
giant spider
centipede
scorpion
t
Trappers et al
trapper above
lurker
u
Equines
black unicorn
white unicorn
gray unicorn
pony
horse
warhorse
v
Vortices
fog cloud
dust vortex
ice vortex
energy vortex
steam vortex
fire vortex
w
Worms
baby long worm
baby purple worm
long worm
purple worm
x
Grid bugs plus one
grid bug
xan
y
Lights
yellow light
black light
z
zruty
'
All golems
@
Humanoids
All humans
Medusa
The Wizard of Yendor
high priests
aligned priests
Croesus
All elves
Lycanthropes in human form
doppelgangers
Guards
Soldiers
Immaterial Undead
ghost
shade
&
Demons
water demon
horned devil
succubus
incubus
djinni
sandestin
erinys
barbed devil
marilith
vrock
hezrou
nalfeshnee
pit fiend
balrog
Yeenoghu
Juiblex
Asmodeus
Demogorgon
Orcus
Geryon
Dispater
Baalzebub
Death
Pestilence
Famine
:
Lizards
;
Terrors of the Deep
giant eel
electric eel
kraken
piranha
jellyfish
shark