So I am a newb at this game..trying to learn it. On my 2nd game (first one got stomped on) and I am doing ok. However I cannot figure out how to build mounted units.

I have researched mounted warfare and I have built stables on several horse pastures on land I own. I have over 50 horses according to the bar at the top of the screen that shows your resources. However no matter where I look I cannot build any horse units.

I also tried tried to design my own and it doesn't have any listed there. What am I missing?