Hi, this as been raised before.
It seems it is not only a problem of horses not beieng in the first 4 resources produced in a province.
In a PBEM I have Navara producing only Agriculture and Textile. This gives me +8 Golds, +6 foods,+4 horses, +2 Wool, +1 Wine, +1 Labor. So on the trade window I should get Golds, foods, horses and wool.
Unfortunetely here is what I get:
Gold, food, Wool, Wine...
I guess I can ask my trading partner to ask the trade, maybe the problem will be taken care of then...Because Horses do show for other nations if I ask them to trade with me.
I got screen shots and saves if you want. But it's a PBEM game.