So, after you have found out what a THING weighs, and how many horses it will take to carry the THING, go back to the unit stats and look at the landscape stat tab. This will tell you how many action points it will cost for the THING to enter a giver terrain type. Then, COMPARE the THING to a horse. (There is a button on the unit stat screen for this) This will tell you how much faster the THING will move if you put it on a horse (or behind, I suppose) and weather it's worth the cost to horesorize (horesify?)the THING. And you are right, in some terrain horses may be slower that the THING. There is a lot of these interacting values in this game. If you need to know about something to predict how something else will mesh with it, the information is probably somewhere. The interface, however is not the best organized in the genre, so CLICKING on everything, READING and UNDERSTANDING what pops up, and REMEMBERING where you found it will pay off in your future moments high drama, crisis and final triumphs.
hope this was both entertaining, and helpful.