I have created the "DismountEx" Patch.
Please update FEBuilderGBA.
You can ride a horse with the Rade and Exit command.
For riding units, please create a setting.
By default, Eirika(Lord), Seth, Neimi(Archer) can ride a horse.
In addition, you can forcibly put it on a horse with an event order, and you can lower it.
However, using this function affects promotion, so it would be better to target only promotion unit.
Or you need to adjust the mechanism of promotion.
For compatibility, when selecting Archer to ride on the ballista, the ballista is given priority.
First of all, please try after back up.