Kuiper Belt Object (486958) 2014 MU[SUB]69[/SUB], nicknamed "Ultima Thule" by NASA in homage of the ancient mythical island at the farthest end of the world, is tiny - only about 20 km across at its widest. So, only slightly larger than an asteroid station or maxed-out Coriolis. Nowhere near big enough to be classed as a "planet" by ED, though it might fall into the still-invisible "comet" category.
As you can probably tell by that rather large six-digit number at the front of its name, "Ultima Thule" is just one of hundreds of thousands of similarly-sized objects in the solar system. ED can't be expected to model every single one of these.