A Thwimp is the smaller relative of the Thwomp, first appearing in Super Mario World. Their way of attacking is jumping towards Mario, trying to hurt him. It is indestructible except with a star, and probably because of their smaller size, instantly kills Mario with one touch, so beware. It also appears in New Super Mario Bros. U.
 Johnnie's tips
If you have the chance, run as fast as you can, because some of the levels this appears in, they fall in a domino sort of effect, forcing you to run.