That something (and you can see it a bit better in the video below or get a better view in a daylight picture over on Teslarati or even more at TMC) makes the car a mystery. Speculation is basically centered in three camps: either it's a weight thing, to allow for Tesla to test out the all-wheel drive system that is coming in the Model X and also to the Model S; or maybe it's test equipment for autonomous drive experiments. The third likely guess is that the blocky item is simply meant to simulate the weight and drag of a Model X and allow Tesla engineers the chance to test the suspension and handling of the upcoming falcon-winged crossover.
Officially, there's no clarification on what, exactly, we're looking at. We've asked Tesla for a comment (and we're not the only ones), but have not heard anything back. If we do, we will of course let you know. Until then, we'll join Teslarati in calling this the Model S/X and wait to see what information others can scrape together. What's your guess?