Rampant means wild, out of control, to be up on your hind legs roaring at the world. You don't have to be four-legged: "The children ran rampant at the supermarket even though their mother tried to control them."
If you're running rampant, you're on a rampage. Both come from the French word ramper meaning "to climb, creep" like an animal on hind legs, paws in climbing-mode, or like wild plants such as the kudzu that ran rampant over an old barn until the entire thing was covered. There can also be rampant wildfires that destroy houses in the Southwest, or robots that run rampant in the lab after the janitor accidentally sets them free.