A nasty normally refers to a monster that can be summoned by means of the summon nasties monster spell:

• c cockatrice
• H ettin
• E stalker
• H minotaur
• D red dragon
• D black dragon
• D green dragon
• Y owlbear
• w purple worm
• T rock troll
• x xan
• g gremlin
• U umber hulk
• V vampire lord
• X xorn
• z zruty
• @ elf-lord
• @ Elvenking
• D yellow dragon
• q leocrotta
• q baluchitherium
• Y carnivorous ape
• H fire giant
• A couatl
• @ captain
• g winged gargoyle
• h master mind flayer
• E fire elemental
• J jabberwock
• L arch-lich
• O ogre king
• T Olog-hai
• ' iron golem
• j ochre jelly
• P green slime
• R disenchanter

There also exists an M2_NASTY monster flag, which has an influence on the experience the monster gains and what weapons they might be generated with, but is otherwise independent from the above description of nasties:

• h mind flayer
• h master mind flayer
• w long worm
• w purple worm
• J jabberwock
• Y owlbear
• Z skeleton
• @ high priest
• @ Wizard of Yendor
• @ Croesus
•   shade
• all angelic beings
• all adult dragons
• all large humanoids
• all vampires
• all major demons (including lords and princes)
• the Riders
• the Grand Master
• all quest nemeses (except the Master of Thieves @)

SLASH'EMEdit

In SLASH'EM, the following monsters are also considered "nasty" for purposes of the summon nasties spell.

• V, vampire mage
• :, rhaumbusun
• :, basilisk
• c, pyrolisk
• o, demon orc
• q, mastodon
• w, rot worm
• o, snow orc
• D, silver dragon

The following monsters also have the M2_NASTY flag set.

• a, tsetse fly
• n, quickling
• q, catoblepas
• s, carrion crawler
• w, rot worm
• z, kangaroo
• J, vorpal jabberwock
• P, shoggoth
• P, giant shoggoth
•  , shadow
• :, rhaumbusun
• all new angelic beings
• all new adult dragons, including wyverns D and hydras D
• all new vampires
• all new major demons, except spined devils
• all new quest nemeses
• Nightmare u, Beholder e, and Vecna L
• all other new unique monsters (except One-Eyed Sam)