Digga-leg is a robot that appears in Super Mario Galaxy 2. His birth is unkown as he is a robot and it has only appeared in Super Mario Galaxy 2. If you get confused, you might be looking for Megaleg.
Super Mario Galaxy 2
Digga-leg appears in Super Mario Galaxy 2 in the spin dig galaxy. You have to defeat him twice because there is a daredevil comet after you get the comet medal. You have to hit three times and then the star will smash out of the glass, letting you take it. But watch out, he has a few minions that he'll release. These enemies are called Diggas. The daredevil run is much harder, as you must beat him with one life. If you've beat him congrats. If you haven't played the game yet, beware because Digga-leg is very fierce!