You also have to research the Warfare Tech "Mounted Warfare" or something in the line.

love this song. always play it at open mic nights.

YOu need to do three things:

1) find a horse resource, there's not too many of them & build a stable on the resource .. you should start seeing horses in the resource screen at the top.

2) research military tech that allows you to use the horses you've found

3) Design a unit that uses horses. I have a horse archer class. You should also be able to buy horses in the item shop (although I don't know if this is tied to the tech or to availability or both). Horses show up in the item shop and you can buy them at 100 a pop, overpriced but not as much so as some other things are.

sovreign and champions don't need the resource only the tech researched. If you want to make regular troops on horses you need the resource.