Horses being stacked next to one another should be enabled. They have no real fighting capability so it wouldn’t hurt to have them stacked closer when they aren’t being used. Especially if you are attempting to store them in a stable like fashion (see pic)
visually looking at the picture makes me agree, as each horse has to be leveled and have a saddle on top of that, it seems rather silly that the empty space is there and only really encourages having a bigger build for no reason
Place the horses in the pattern shown in your picture and set them all to guard.
stall #1 Horse
stall #2 Empty
stall #3 Horse
stall #4 Empty
stall #5 Horse
stall #6 Empty
stall #7 Horse
Get on the horse in stall #1 and ride it away. Get off and tell it to stop following you which puts it in scouting mode
Repeat this for the other horses
You can now put horses in stalls #2, #4 and #6 in guard mode.
Walk away and do something else. The horses which you’ve ridden away will still be in scouting mode and eventually return to their guard locations in stalls #1, #3, #5 and #7
You now have horses in each one of the stalls.
I know this is a messy way to do it and and you should be able to place them as you suggested without using this workaround but I don’t know how widely know this method is.
Not sure where you’re playing, but for single player and dedicated servers, there is a rather obscure setting that controls how close you can place Thralls, Pets and Mounts together (it isn’t exposed in the in-game server settings UI anywhere), but anyone playing Official Online is out of luck aside from using the trick OldManFromTheNorth described.
With the game closed (or server running the session closed), check out:
ConanSandbox\Saved\Config\WindowsServer\ServerSettings.ini
Find:
ThrallExclusionRadius=500.000000
Set to 200.00 or so and you’ll have no problem placing those horses the way you want.