Mounting Spells. How to?

[:1]Okay, so im well pretty new to this WC3 Map making stuff, even though i got a slightly amount of experience in making custom units, spells and triggers. But i've stumbled into a problem right now while starting my first map project (And that sadly too early):

I Cant figure how to make my footmen mount the "Riderless horse" units. I am using the hippogryph mounting spell obviously, but i cant seem to make it work. What can i be doing wrong?

Thank you alot for helping me |||The hippogryph taming ability should work, remember that you need to give the footman, the riderless horse and the knight (horse+footman) an ability! And i thought that you needed different abilities too, so just check how its done with the archer, hippogryph and the hippo-rider ;)