Well, I think I have figured out the free black arabian horse in Saint Denis. None of my friends have gotten it, but I have gotten it on both my playthroughs and 2 more times using saves. The one thing I'm not sure about is if you need high honor or not, I've always been at the early game cap (you can't hit maximum honor until near the end of the story). There is 2 robbery events at night. The first is a guy being held up in an alley, the second is someone tries to mug an arguing couple. Every time I've shot that second mugger, I look around and there is a black arabian horse just chilling out - though the first time I didn't notice and she walked up to me. You can pretty much get it right away, I got it last night at like 14% completion.
Edit: I think this is the best horse you can get until the epilogue (though later in the story you can just buy a black arabian). It is 1 health and 1 stamina better than the wild white arabian horse than can be tamed.