Edit: Looks like I jumped the gun. Thought I had found something to share. This seems to actually be tied to Fire and Brimstone being toggled on.
I've been lurking on the site for awhile and thought someone might enjoy this.
I was doing the new content in Northern Barrens when I noticed that my Dreadsteed was glowing green. It seems that now when you have enough embers and are glowing green your mount will as well. This effect was pretty cool. I've been able to replicate it a few times so far. Not every time so I'm sure it's not intentional.
Here is a vid I captured. Not the best resolution but you can see what was going on. Hope someone else gets some enjoyment out of this.