The permission is talking about stashes, but horses use saddle bags. I assigned the permission, but I don't see any extra storage.
Am I missing something? I expected more slots in horse storage.

vehiclestorage.ridablehorse.1stash -- 1 x 42-slot stash (42 total capacity) 
vehiclestorage.ridablehorse.2stashes -- 2 x 42-slot stash (84 total capacity)​

This plugin does not affect the built-in saddle bags since they are too limited. Like with other vehicles, this plugin adds extra stash containers onto the horse.

Ok thanks.

Are they visible? Where should I see them? Where are they accessible from?

You will see them on the hips. Players must access them directly. The positions are configurable as with all containers.

Awesome thank you. Great job on the plugin btw.