Monstars are an enemy in Braid. Typically they walk back and forth, and do not attack Tim directly. If Tim touches them by walking into them, or they land on him by chance, Tim "dies" (see Time Manipulation).
Tim can kill Monstars rather simply by jumping and landing on top of them. Doing this bounces Tim back up, actually higher than his initial jump. In turn, bouncing on two or three Monstars after each other propels Tim even higher. This technique can be used to reach otherwise inaccessible heights.
Most Braid players refer them as Goombas since they look like the Goombas in Mario series. In fact, they are hedgehogs, another possible reference to Sonic series.
They also resemble the Knight near the end of the game.