Banelings and the baneling nest are exclusive to Zagara. She unlocks additional upgrades for them at Level 11. Once Zagara reaches Level 7, all baneling nests are upgraded with birthing chambers that cause them to automatically spawn banelings regularly.
The following section contains information from a previous version of StarCraft II which is no longer valid.
The initial game mechanics of the baneling were simple: an upgrade was researched at the spawning pool which allowed zerglings to morph into banelings. Later design decisions thoughtfully concluded that there should be a building to visually indicate the technology available to a zerg player. Thus, the art team developed the baneling nest The "nest", as designed by Jesse Brody appeared prior to WWI 08.
The final art pass, which deviated from the old "nest" concept, was carried out by Phill Gonzales. He was interested in making a large, bloated baneling "brood mother" planted on the creep, too fat to move and instead serving as a building. Samwise Didier however, liked Brody's old concept and made the baneling brood mother be attached to the main growth of the building, rather than being a building in itself.