Kavious is right there--horizontal hotbars have to be done through UI modification outside the game (i.e. in the UI files inside your install folder, or by downloading someone else's modifications) and then loaded using the in-game command.
I really like some of the community created UIs out there (I use qqui myself), but if you're savvy with that kind of thing and you want to keep the regular UI instead of using someone else's version, I believe you should be able to just modify the regular UI file to have the hotbar be horizontal. You probably want to make a copy and modify that though
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