There are three approaches to having horizontal error bars in Prism.

XY graphs with X error values

In the Welcome dialog, or using Format Data Table, you can ask Prism to create a subcolumn for X error bars.

Prism will then plot each point with horizontal X error bars sticking in each direction a distance defined by the X error value you entered.

Rotate Column or Grouped graphs so the Y axis is horizontal

A choice on the third tab of Format Graph, lets you flip any Column or Grouped graph so the Y axis is horizontal. If you plot symbols or bars with error bars, those error bars will now be horizontal, but that horizontal axis is now the Y axis.

Depth charts

When you place an XY graph on a layout, you can choose to have it rotated 90 degrees to create a depth chart. Error bars that are vertical on the graph will now be horizontal on the layout.