When the prophet Malzahar was reborn in Icathia, he was led there by an ominous voice which thereafter anchored itself to his psyche. From within, this voice bestowed upon him terrible purpose, and though Malzahar was no longer tormented by its call, the voice did not cease its unrelenting summons. This baleful beacon's gentle flicker - now fastened to Runeterra - drew forth a putrid beast that ambled across a threshold it did not understand, widening a fissure between spaces which were never meant to meet. There amongst the haunting ruins of Icathia, Kog'Maw manifested in Valoran with unsettling curiosity. The spark which led him to Runeterra teased him still, urging him gently towards Malzahar. It also encouraged him to familiarize himself with his new environment, to the stark horror of everything he encountered on his journey.
The enchanting colors and aromas of Runeterra intoxicated Kog'Maw, and he explored the fruits of the strange world the only way he knew how: by devouring them. At first he sampled only the wild flora and fauna he happened across. As he traversed the parched Tempest Flats, however, he came upon a tribe of nomads. Seemingly unhampered by conventional rules of physics, Kog'Maw consumed every nomad and any obstacles they put in his way, amounting to many times his own mass and volume. The most composed of his victims may have had time to wonder if this was due to the caustic enzymes which stung the ground as they dripped from his gaping mouth, although such musings were abruptly concluded. When his wake of catastrophe reached the Institute of War, an enthusiastic Malzahar greeted him with an enticing prospect: taste the best Runeterra could offer…on the Fields of Justice.
"If that's just hungry, I don't want to see angry."
Kog'Maw was first featured in a Screenshot of Malzahar, though no further information was given at that time. Relating the Sneak Peak screenshot and the Malzahar screenshot, however, and you can see clear similarities.
On June 18th, 2010 a new preview was given by Pendragon which finally cleared information up:
Those of you who were paying attention when we released Malzahar might have noticed somebody lurking in the background. And today you get to put a name to that face… err… mouth. Meet Kog’Maw, Mouth of the Abyss. Now many of you have been asking for more monsters in the League of Legends lineup, so if you had any doubts about whether or not we’re taking your suggestions to heart, this gentleman… err… gentlemouth should assuage those concerns.
Bio-Arcane Barrage: Kog'Maw's attacks gain range and deal a percent of the target's maximum health as magic damage.
Void Ooze: Kog'Maw launches a peculiar ooze which damages all enemies it passes through and leaves a trail which slows enemies who stand on it.
Caustic Spittle: Passive: Increases armor penetration. Active: Kog'Maw launches a corrosive projectile which deals magic damage and doubles his passive armor penetration for 4 seconds (Removes the passive until available again).
Living Artillery (Ultimate): Kog'Maw fires a living artillery shell at a great distance dealing damage and revealing targets. Additionally, multiple Living Artilleries in a short period of time cause them to cost additional mana.
Icathian Surprise (Innate): Upon dying, Kog'Maw starts a chain reaction in his body which causes him to detonate after 4 seconds; dealing magic damage to surrounding enemies.