It's kind of hard to explain what this is without spoiling a major plot point of Mother 3...This angel-thing is part of a boss battle that was removed from the game because Nintendo thought it was too disturbing. I think I read somewhere that even the programmers of the game were freaked out by it.

Here's a video of the boss but remember that there are spoilers, so don't watch it if you haven't played the game yet and don't want to know what happens!

http://www.youtube.com/watch?v=wqjFXTe6-6A

The angel appears around 0:55.
You can hear a voice in the background saying "Lucas...Lucas..." O_o