Those little walking Pirhana-Plant-like things in the Mario series. Although the term "Chompweed" was first coined in Super Mario RPG: Legend of the Seven Stars, it is save to assume that they are the same creatures that appeared in Super Mario Bros. 3 and Super Mario World, where instead of being white, they were black and they sat in one place. You needed either Kuribo's Shoe or Yoshi to cross them.
Chompweeds can harm you in the Mario games. In Super Mario RPG, however, they steal your coins. Little bastards.