Looks like everyone have missed this one, time is short if you want to add this to your Crusader kings DLC library.
There’s apparently 2 different horse lord DLCs, an expansion and a content pack, and it’s the expansion one that’s free. So if you want to lord it over horses, here’s your chance.
.Expansion - Crusader Kings II: Horse Lords on Steam
Plain link in case the widget doesn’t work for people.