I have two sons, so I am woefully ignorant of the whole My Little Pony phenomena (among so many other things, as my wife reminds me – often). Given that, I had no idea who Princess Celestia was until after I was exposed to the following video. As I now understand it, Princess Celestia is a "Pegasus Unicorn" and co-ruler of the magical world in which the My Little Pony stories are set. Significantly, at the end of each show, one of the ponies writes a letter to Celestia, reiterating the moral of the story for the episode.
Now, although I was clueless about My Little Pony, I was definitely well aware of the identity of GlaDOS and Portal. I assume that pretty much everyone here at GeekDad is similarly familiar with the game, so let's just get right to the video of Ellen McLain, the voice actor responsible for GlaDOS:
As you can see, Ellen is a fan favorite at gaming conventions, and a great sport overall.
The audio for the video has been tweaked to make Ellen's voice sound like GlaDOS, but if you're interested, the original video is here. The blurb for the original video includes this information about the entire event: