Halo 3: ODST's Engineers were a surprise for me. I'd never heard of these creatures before - plenty of fans had though and Bungie made no secret of their existence:
"First seen in still images prior to the retail launch of Halo: Combat Evolved (and then again by industrious players who coaxed character models out of unplayable portions of the disc), our official unveiling of the Engineer didn’t go down until eight years after the Master Chief first went toe-to-toe with the Covenant. But since you’ve now seen these curious, armor-clad characters floating and sputtering around the city of New Mombasa, we figure the time is right to declassify their artistic origins."
So now that Halo 3: ODST is done and dusted, Bungie took the time to explain a bit more about the design and origin of the Engineers. A chat with the lead designer Shi Kai Wang sheds a lot of light - for instance they were inspired by sea creatures like jelly fish!
"I definitely looked at a lot of sea creatures for inspiration - the way they move, the way they emit light, the fluidity of their bodies - and tried to translate that into a creature out of water. Under water sea slugs and glowing jellyfish were the main source of inspiration." - Shi Kai Wang.
Due to time constraint issues, the Engineers were "left on the cutting room floor" while Halo CE was being finalised. They were cut so late in the piece, however, that they remained in the game's code. Xbox modders and hackers located the Engineer files within the code on the game disc and manged to spawn Engineers in various configurations within the game. These are the images that may be found on the internet:
What's purple and doesn't look like Cortana?
Will the Engineers be in Halo: Reach? There's a real possibilty they might be as they are actually referenced in The Fall of Reach novel. The Master Chief watched an Engineer dismantle a vehicle's engine, assemble it into various other working configurations, and then return it to its original state (page 183 of the book).
There is real potential for at least some kind of reference in the xbox game now that they are official in the 'released game cannon' and not just in the Halo novels. Time will tell...
Extra for Experts: Maybe Microsoft should have used the Engineers to fix all the Red Rings of Death that seem to occur all over the world....