The history of Hack is just as convoluted and chaotic as for any other roguelike.
The first version of Hack (Original Hack, or Jay Fenlason's Hack) seems to have been written around 1982 by Jay Fenlason, with the help of Kenny Woodland, Mike Thome and Jon Payne. Nothing is known about this version, but it seems to be an implementation of Rogue with added monsters and items.
There are three variants of this original version known today:
(1) hack121, a DOS executable without sources dubbed (downloadable here);
(2) PDP-11 Hack, a version for the PDP-11, published on Usenet in February 1985 and ported to a UNIX clone (PC/IX) in May; and finally
(3) Hack 1.0 by Andries Brouwer, a heavily updated and extended version of the original Hack, posted on Usenet in December 1984.
Hack 1.0 is the first well-documented ancestor of the Hack/NetHack genealogy, and it already features different character classes, a pet dog, shops as well as more and new types of items and monsters.
During the following months, Brouwer published three updated versions on Usenet (1.0.1 to 1.0.3), all uploads being plagued by loss of data and other misfortunes -- many sites just could not cope with a massive upload of over 400 KB of data back then...
The version 1.0.3 then became the basis for several DOS versions (PC Hack), ported by Don G. Kneller, one of which (V3.51) in turn ported to the Atari 520/1040ST by R. Black (ST Hack).
This plethora of versions was then merged together by Mike Stephenson to produce NetHack, which is still in active development today -- but that's another story...