StarCraftDefinition
WoWWiki defines creep:
A creep is any monster in the game or any non-quest-giving and non-shopkeeper NPC combatant.
Typically used on Battle.net on Warcraft III. The term mob is more often used in World of Warcraft.
Comes from Warcraft III where the neutral (but aggressive) packs of monsters would guard certain areas from your armies. Originally, in Starcraft, creep was a layer of slime where zergs could build similar to blight.
As far as I can tell, creep was first used as a type of neutral creature in Warcraft III "to make its maps seem more like a world, rather than a game board. This will create an immersive atmosphere as well as create unique strategies and tactics."
World of Warcraft, 2004
The earliest definition in Urban Dictionary is from 2004 for Blizzard Entertainment's World of Warcraft:
Creeps
New terminology for MOBs that Blizzard is trying to instill in the World of Warcraft playing community.
Character 1: Let's go kill that 'creep'.
Character 2: WTF is a 'creep'?
Character 1: Any bad guy we can kill.
Character 2: It is called a MOB you fucking looney tunes bitch.
by Mangina2004 Jan 30, 2004
WarCraft III, 2000
The term was used by Blizzard on their official preview website for Warcraft III: Reign of Chaos (aka War3 or WC3 or RoC, released 2002).
The oldest archive of http://www.blizzard.com/war3/neutrals/creeps.shtml is from 15th December 2001:
The Blizzard Warcraft III preview homepage announced the creeps on December 14, 2001:
1 2 | 1 4 | 0 1
Warcraft III Creeps Revealed
"Even the most expert travelers must be wary, for in the exploration of any map they're bound to encounter a variety of Creeps." Learn more about these units on the new Creeps page, currently featuring the Ogre Lord.
Earlier, it was used in 2000 Usenet in a Warcraft III preview.
"news [ÎÅÊÔÒÁÌØÎÙÅ ÓÏÚÄÁÎÉÑ] -war3-" - fido7.ru.game.blizzard - Ildar Ahmetzhanov - May 13 2000:
The Infoceptor ÏÂÎÏ×ÉÌ %subj'Å×ÕÀ ÓÔÒÁÎÉÞËÕ: (http://www.infoceptor.com/information/war3/miscunits.shtml)
Warcraft 3 will feature a wide range of neutral creatures to make it_s maps seem more like a world, rather than a game board. This will create an immersive atmosphere as well as create unique strategies and tactics.
...
Creeps:
Creeps are wondering citizens of Azeroth, littered across the map. Upon encountered by players, they have different reactions depending on the players race and power. Some may attack outright, while other may offer their services of money, while other_s will give specific quests. Creeps are always armed, and hence are sometimes a threat, especially to fledgling forces.
StarCraft, 1997
There was creep in StarCraftBlizzard's StarCraft which was released on 31st March 1998, but this was a different thing altogether. Examples can be found in Usenet in 1997 and 1998Mentioning it here for completeness only.
"The UBERLIST" - alt.games.starcraft - David Jarrett Fate quoting Blizzard rep Pat N. - Nov 14, 1997:
ThisBut this isn't quite the same thing as a neutral unit. From the StarCraft Wiki:
At some point, creep colonies were present in the game engine of StarCraft II. However, the creep colony was replaced by the creep tumor, and the defenses with the spine crawler and the spore crawler respectively.
World of Warcraft, 2004
But perhaps it evolved in other games from this starting point, and it at least had this meaning in this Urban Dictionary definition from 2004:
Creeps
New terminology for MOBs that Blizzard is trying to instill in the World of Warcraft playing community.
Character 1: Let's go kill that 'creep'.
Character 2: WTF is a 'creep'?
Character 1: Any bad guy we can kill.
Character 2: It is called a MOB you fucking looney tunes bitch.
by Mangina2004 Jan 30, 2004