The Water Horse riding horse is lovely, but the only way to ride it is astride. Consequently, many Victorian ladies were afraid they would scandalize the neighborhood if they bought one. An official sidesaddle is being worked on, but now there's a hud that puts you in the right position. The actual saddle doesn't change, but no one will notice if you wear a big enough skirt.
It works for both the warmblood and the quarter horse. The right stirrup seems to be rigged to the right leg, and it stays with the leg as it crosses over the horse. You can edit the stirrup to make it transparent, which makes both stirrups disappear. Then, if you want a side saddle stirrup, you can get one that attaches to your foot, so it's always in the right place.
Why would you buy this hud and an extra stirrup when an official sidesaddle is on the way? When you're impatient and these add ons are cheap! You can get both for $57L.
Water Horse warmblood
Gaslight riding habit in red from Ladies Pleasure