We've seen large My Little Pony plushes before, such as this Princess Celestia large enough for a child to ride and this huge Pinkie Pie.
But these ponies by Xion Munro exceed in size even those impressive works. She operates on commission and is no doubt very busy trying to keep up with demand. Look at that Luna on the couch. I've never been much of a Luna brony, but now I need one, too!
I see no practical reason why it would not be possible to build a pony plush as large as a house. And, obviously, if such a thing can be done, it should.
-via Equestria Daily