The idea of large planets existing well beyond Neptune is no new idea, it has been around ever since Neptune was discovered. With Voyager 2's recalculation of Neptune's mass, the hunt for an extremely large "Planet X" dwindled away.
Today, the story of that there might be a "Planet X" and even a "Planet Y" appears again, but this isn't something really out-of-the blue. Space is big, objects that don't emit light are dark, and telescopes only have certain attributes making them good for certain tasks. I have also seen too much claims that one of these planets is Nibiru(which the Sumerians never thought existed as a planet, more of a crossing point) or some other fanciful thinking.
Here is the abstract from the Scientific Paper: Link
This blog/podcast does a good job addressing much of the Planet X myths and realities among other topics.
Also submit hypothetical planet names.