How would I do this? Need help.

I'm making an rpg and I would like to use horses as transport units but instead of making the unit go inside the horse i want it to ride the horse like the archmage does

i don't know much about horse triggers but check the Spells section and type in mounting tutorial and maybe you could use one of those they arent perfect but they've worked ok for me in my maps.

create a spell mount (you could base it on morph ability and set the morphing unit to horse then make art attachment in the spell 'the rider' if that dont work create a special effect on horse mount of a unit however it'll look pretty pants as the model wont move with the horse like the arch mage. you you want full animations and the look of a ranger on a horse (for example ) then go search this site for some there are plenty of mounted units to choose from.