The World XY plane is “horizontal”, and the World Z axis is “vertical” and immutable. However you can establish local CPlanes (coordinate systems) in any orientation and in any viewport. What you should know is that each viewport by default has its own coordinate system, the standard parallel/ortho viewports Front, Right etc. have by default local XY oriented horizontal/vertical to that viewport. By default the Perspective viewport has its CPlane the same as World Top.
If you’ve built some stuff and rotated it off axis and don’t know where you are anymore, there are two main ways to get back to “known space”:
Multiple rotations using object snaps and Ortho - use the Rotate command in say, the Front viewport choose a center of rotation and a point both along an edge which you would like to be “horizontal” and rotate the object until Ortho "snaps’ it to the horizontal. Do the same from the Right viewport, then finally from the Top viewport if necessary.
Set the perspective viewport to World CPlane (if you have changed it), then use the Orient3Pt command, pick 3 points on the object which you want to fall on the origin, X axis and Y axis respectively, then pick 3 corresponding points where you want the object to “land” in the World CPlane.
Quickie video of both methods. Note that in the first method, the order in which you do the rotations is important.