A horse-bug would be a strange creature indeed. But that is not what I am here to talk about!
This bug is as consistent, repeatable, and long-standing as the others I've brought up, and I have a pretty illustration just to show it. While I feel kind of odd making such a large number of posts as of late, I'm illustrating as many issues as I can when they crop up if they are not in the Known Issues list or otherwise confirmed as being worked on or known.
So let's begin.
When you make a unit that's mounted on a horse, the unit card displays just fine when you're in the creation dialogue page for it. However, once the units are actually completed, available for production, and produced, the unit portrait does some odd things. Observe:
"Whoa there, boy!"
It seems the proper animation pose does not display on the person riding the horse, so it looks like the person in question is being violently thrown off by his mount.
To reiterate, when MAKING the unit card, it displays properly. It's only once it's produced and re-referenced.