Smoke Monster

"Evil Spirit" (unofficial) was an unnamed monster that Garnet was trying to destroy in the episode "Together Breakfast," until Steven distracted her.

"Together Breakfast"
It escaped from an evil scroll and possessed Steven's Together Breakfast. Gaining all the powers of a breakfast, it pinned down the Gems, broke Pearl's spear, and almost defeated them, until Steven destroyed it along with the breakfast.

Trivia

 * The Evil Spirit resembles Giygas from the Super Nintendo RPG, Earthbound. (Or Mother 2 in Japan.)

Evil Spirit