F4322B313992ECCC6D53642DB2C9D63A461FB55F (NetHackW.exe; 3.67)
NetHack has been available in source form for a long time,
along with binaries that may or may not have debug info included.
To access the game's debug mode, enter the following in the command prompt:
nethack -D -u wizard.
The source code has a handful of monsters mentioned that are not present,
such as the Beholder and the Shimmering Dragon. While code for these does
not get compiled in, their graphics do.