The Forerunners...according to the game's storyline. They built it as a weapon of mass destruction, used to wipe out all sentient life in the galaxy as an attempt to starve The Flood (a parasite that feeds on the flesh of living creatures) and end a war. This could be one variation of the question "Who created Halo?"
The below text is another variation of the question...one that asks "Who created the game Halo and it's storyline?"
This question gets some dispute as to who really created the game series Halo.
Some people say that Microsoft created Halo. These people are incorrect. It was Bungie that created Halo, or should I say that it was Bungie that created the game's story. Microsoft only wrote the gaming code and produced the game, they did not create it. Halo 4 will be created by 343 Industries instead of Bungie.
Also, Bungie is a separate company from Microsoft, although most Halo products say that Bungie is a trademark of Microsoft, they are a separate company in contract with Microsoft.
Halo Reach, Halo: Combat Evolved, Halo 2, Halo 3: ODST, and Halo 3 were made by Bungie; Halo: Wars was created by Ensemble Studios; Halo: Combat Evolved Anniversary and Halo 4 were created by 343 Interactive
Halo effect was created in 1920.
Halo in a Haystack was created in 1994.
Halo Friendlies was created in 1998.
Halo Star was created in 2004.
Halo James was created in 1988.
Dime with a Halo was created in 1963.
The Halo Benders was created in 1993.
Arbiter - Halo - was created in 2004.
Halo - bar - was created in 2004.
Luna Halo was created in 1999.
Devil's Halo was created in 2008.