The grue first "appeared" in Infocom's classic Zork. It proved so popular, the creature was included in most of Infocom's subsequent text adventures in some form or another. Even the non-fantasy games would allude to the grue, as in the mystery game Suspect, which featured a racehorse named Lurking Grue.
In truth, the malevolent beast didn't appear until several years after the first allusion to it in Zork I, when games like Beyond Zork and Journey made the creature more than just a lurking threat. Instead, it spent most of its existence hidden in the darkness, waiting for adventurers who were foolish enough to wander around without a light source. In the early days, you never actually saw a grue - though you were likely to see this room description on more than one occasion:
"It is pitch black. You are likely to be eaten by a grue."
Most Infocom games let you ask questions about various items in the game. Asking Zork "What is a grue?" would elicit the following:
"The grue is a sinister, lurking presence in the dark places of the earth. Its favorite diet is adventurers, but its insatiable appetite is tempered by its fear of light. No grue has ever been seen by the light of day, and few have survived its fearsome jaws to tell the tale."
The grue was invented by Dave Lebling, who actually took the name of the creature from a series of stories by fantasy writer Jack Vance. But Lebling only took the name from Vance. The idea of the lurking beast who crawls through the darkness was all his own.
The grue was actually a gameplay device, and a somewhat cheap one at that. It was simply a means of keeping you from moving around without a light. But it was a sinister beast nonetheless, made more so because its appearance was left entirely to the imagination. It's no wonder the grue became a staple in Infocom games and that its horrible visage, or lack thereof, remains in our memories more than a decade later.