A grid bug, x, is the weakest monster that appears in NetHack. A grid bug has a monster level of 0 and a difficulty of 1 - its sole attack is a single, weak bite that zaps you, but is too weak to actually damage your inventory. They have shock and poison resistance.
As indicated by a pair of true rumors, grid bugs are unique in that they cannot move or attack diagonally - this also applies to you if you are polymorphed into a grid bug. Some have proposed moving like a grid bug as an unofficial conduct.
Grid bugs are easily handled by all but the most unfortunate beginning characters - they move at the same speed as you, but their directional restrictions and weak attacks ultimately serve to quickly introduce players to the strategic benefits of diagonal movement. Grid bugs usually deal no damage at all with their bite, and are usually nothing to worry about once you reach or pass experience level 2; if you need to train up a weapon skill, they can be useful as target practice. Otherwise, grid bugs are best left as fodder for a pet you want to train, since they gain at least one maximum HP per kill as opposed to the pitiful single experience point you will get.
Of note is that if a grid bug manages to successfully kill another monster (e.g. while under the influence of an early source of conflict) and gain a level, the shock damage from its bite becomes capable of destroying rings and wands in a target's inventory.
The grid bug first appears in NetHack 3.0.6.
The concept of gridbugs originates from the Tron franchise, which began with the 1982 Disney film Tron. Despite this, gridbugs themselves only make a brief appearance in the original film; earlier drafts indicate there was going to be a major scene featuring a gridbug swarm as a significant threat, which did not make it into the final script. Gridbugs are digital creatures that cause glitches and often appear in swarms, and are capable of "derezzing" programs and causing havoc throughout an entire system; their size varies depending on the portrayal, but they are usually depicted as four-legged "insects".
Gridbugs feature more prominently in the Tron arcade game from 1982, which was in development before the script was finalized: they appear in the I/O Tower sub-game, where the player must guide Tron to the flashing circle of an Input/Output Tower within a set time limit while avoiding or destroying gridbugs. Their inclusion in NetHack plays off the idea of the player character as a "hacker", which refers as much to programmer and geek culture as it does to the "hack 'n' slash" genre.
- You can't get there from here...
- You attempted to move or attack diagonally while polymorphed into a grid bug.
In ZAPM, grid bugs are included as a tribute to NetHack, and have largely the same stat block.
These electronically based creatures are not native to this
universe. They appear to come from a world whose laws of
motion are radically different from ours.
Tron looked to his mate and pilot. "I'm going to check on
the beam connection, Yori. You two can keep a watch out for
grid bugs." Tron paced forward along the slender catwalk
that still seemed awfully insubstantial to Flynn, though he
knew it to be amazingly sturdy. He gazed after Tron, asking
himself what in the world a grid bug was, and hoping that the
beam connection -- to which he'd given no thought whatsoever
until this moment -- was healthy and sound.