Create a boarding stable at community lots or your residential lot and let it get used by NPC horses. The horses live at the stable the whole time and their owners will visit them occasionally.

Mod Functionality

The lot trait will spawn three horses. The horses will be

grown up
from unplayed households in your save
mostly the same horses each time so you can build a relationship with them (they might change if Neighborhood Stories or yourself add new horses to the save)

If you want to call one of the horse's owners to the lot you can use the "Call Horse Owner to Stable" Interaction which can be found in the "Horse Care" pie menu. But the owners will also visit their horses on their own from time to time (and might even ride around the stable).

I created this mod because I was really bothered with how empty Duke's Hall felt. You can use it

for more immersion on lots like Duke's Hall or any other equestrian center in your save
to have a place to take your Horse Lover Sims that don't yet own a horse themselves, so that they can learn to ride
to bring more life to your residential ranch and have more horses to care for while not using up household slots
to not feel so bad about all the horses that get adopted by Neighborhood Stories and end up in households that don't even have a stable ;)

The lot trait can be used on any lot type. If you remove it you will have to travel and come back to the lot for the horses to leave.

Plans for the future

(let me know if you have other ideas and also of course if you encounter any issues)

pay your Sim for taking care of the horses on their residential lot
have an "Encourage to mount" interaction on the horses to make other NPCs actually ride
make the horse owners practice on the jumps and barrels autonomously
be able to have some kind of designated riding arena on the lot

Limitations

If a horse is neglected while at a boarding stable they are at risk of being taken away from their family! So make sure their needs are met (it's not that difficult as long as they have enough food).
Flapjack's owner Sienna Grove can't be invited to the stable because she is a special NPC and treated differently by the game
although the horse owners will mount their horses and try to go for a ride around your lot this does not work perfectly and sometimes they will just dismount directly; it works best on lots with a lot of open space
I noticed an issue where the boarding horses sometimes just disappear but I'm still not sure when and why and will have to investigate further; if you encounter this issue traveling and coming back fixes it

Patch For More Horses

I kept the number of boarding horses low in order to not run into issues with the maximum number of Sims per lot (which is 20 without additional mods). If you are willing to use a mod like MCCC to increase the number of Sims (and your PC performance allows it) you can check out Pure_Winter's patch for my mod which spawns more horses for breeding purposes: https://curseforge.com/sims4/mods/pw-stud-farm-lot-trait-more-horses-boarding-stable

In case you encounter any issues (for example disappearing horses or NPC not working) please test again only with this mod before reporting any issues :)

Packs Required
You need Horse Ranch of course.

Installation
Unzip and drag both the package file and the script file into your Sims 4 Mods directory.

Compatibility
I had to change or fix some interactions regarding the horse items and the mount and dismount interactions. So there is a risk that this mod won't be compatible with other mods that change interactions around horses. But I made sure that any changes regarding autonomy are only active for boarding horses at a boarding stable, so it does not affect your game on any other lots.

Translations

I did the translations myself with the help of auto translations, so I'm sure they are not perfect. Please let me know if anything is wrong or weird.

Boarding Stable Lot Trait (NPC Horses at Your Lots)