The fastest way to close lots of pages at once in Horse Browser is to close the Trail they're inside. Better bulk-close controls are on the roadmap; for now, this is the move.
Closing a whole Trail
Make sure the Trail only contains pages you actually want gone, drag anything you want to keep into a different Trail first. Then collapse the Trail and click the X next to it.
Horse asks you to confirm. Tick to proceed, cross to cancel.
If you change your mind right after, How To Undelete Trail covers bringing it back.
