I was wondering, is there some kind of slider I could use to animate a cube appearing out of middair from one side to the other? 
the phrase “from one side to another” is a bit confusing to understand what you expect exactly.
due to just object appearance out of nowhere it seems reasonable to use a material transition from transparent shader to a final one. then the animation to be controlled via key framing of a factor value within a mix shader
What I’m picturing is a cube appearing as if a separate plane were moved along either the x or y axis, each side of the plane determining of the cube is invisible or not.