Got a packaged workbook you can share that demonstrates this?
My initial guess was that you had dragged a horizontal container onto the dashboard, but when you inserted objects into it, you inserted them vertically, and Tableau just kept the name "horizontal" for the container. But when I tried doing that (on 9.0), Tableau reassigned the container to vertical designation.
I'm curious about two things:
1) What version are you seeing this in?
2) What did you do to create this condition?
It's not just you on this, the icon is an error in the most recent version of Tableau Desktop. They should still work appropriately though (at least re: how they're labeled).
Here's another thread discussing the issue too Incorrect Layout Container Icon for Horizontal Container
Hey, Zack. I wasn't aware of that! Thanks for the pointer.