Instructions
Open a new game of "Spore" and design a creature with a moveable mouth. Give your creature a jaw and a mouth; it will need both to eat enough to gain omnivore-status.
Eat as much food as possible during the cell stage of the game. Eat any available plant particles by entering the small buds on the green, plant-like objects. Attack and kill the other organisms by using your jaw and other offensive body parts you added to your creature. After you kill the organisms, break them open, remove the meat particles and eat them.
Pay attention to your creature's status via the "History" button located at the bottom right corner of the screen. There is a three-way bar that shows whether your creature is an herbivore, carnivore or omnivore. If the status is leaning more toward the herbivore section, your creature needs to eat more meat. If the status is leaning more toward the carnivore side, your creature needs to eat more plant particles. Keep its status as close to the middle as you can to make your creature become an omnivore.
Keep eating as many plant and meat particles as you can as you complete the cell stage of the game. Once you have completed the cell stage, your creature will become an omnivore if you managed to keep its status in the middle section of the bar throughout the cell stage. As you come to the land stage of the game, you'll be able to select the omnivore mouth parts to complete the transformation into an omnivore.
Reach the Creature Stage as an omnivore to gain the Summon Flock Ability, which allows your creature to summon other monsters to him for socializing and helps it to attack enemy creatures.
Continue into the Tribal Stage as an omnivore to gain the Flying Fish Ability, which enables you to summon the deep sea monster who causes all the fish to fly out of the water. The fish provide an excellent meat source for your omnivore creature.
Progress to the Civilization Stage as an omnivore to earn the Static Bomb Ability, which freezes everything in a city and renders it defenseless. This ability comes in handy when you're about to launch a military attack on a city.
Enter the Space Stage as an omnivore to gain the Generalist Passive Ability, which allows you to upgrade your passive abilities much more cheaply. Your passive abilities are your upgrades to your energy storage, the amount of health your space ship can hold and the cargo hold.