You can use SiteMenu control which wraps RadMenu. When you set the menu orientation (ItemFlow) to Vertical the items will be expanded horizontally. You can take a look at this demo
Then you can set Flow of the parents as described here - Controlling the Layout of Child Items
(item.Level == 0 && item.Text ==
item.GroupSettings.Flow = ItemFlow.Horizontal;
item.GroupSettings.ExpandDirection = ExpandDirection.Right;
item.GroupSettings.Width = Unit.Pixel(150);
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