Cuchulann discovered that dwarves couldn't mount horses in my mod for some reason. I found the problem, I scale the horses so to the PC's race so they don't change size when mounting/dismounting. Because of this an error in the horse scripts became evident:
There error is in the function HorseGetIsAMount in x3_inc_horse.
Line 2795 lists HORSE_NULL_RACE_GNOME twice, one of those should be HORSE_NULL_RACE_DWARF.
-420
EDIT: Remember that if you change an include script like x3_inc_horse you must use the Build option (you can choose to only build scripts) in order for the changes to take effect.