Ancient crufty hardware or software that is kept obstinately alive by forces beyond the control of the hackers at a site. Like dusty deck or gonkulator, but connotes that the thing described is not just an irritation but an active menace to health and sanity. “Mostly we code new stuff in C, but they pay us to maintain one big FORTRAN II application from nineteen-sixty-X that's a real crawling horror....” Compare WOMBAT.
This usage is almost certainly derived from the fiction of H.P. Lovecraft. Lovecraft may never have used the exact phrase “crawling horror” in his writings, but one of the fearsome Elder Gods that he wrote extensively about was Nyarlethotep, who had as an epithet “The Crawling Chaos”. Certainly the extreme, even melodramatic horror of his characters at the weird monsters they encounter, even to the point of going insane with fear, is what hackers are referring to with this phrase when they use it for horribly bad code. Compare cthulhic.