Howdy!
Sorry for the waaaay late reply to this I got a bit busy with things... (Looks AMAZING BTW :D )

So, full disclosure I made this species in middle school cuz I thought it would be a cool thing to do so I didn't really put much thought into it.
Here are my current ideas for this species.

They are primarily carnivores, and while capable of eating some vegetation, they prefer to eat meat whenever avaliable even if it is starting to rot.
Their shadowy manes are a great asset used for protection and hunting, however, their horns that allow them this great power is also their biggest flaw.
This is due to how clearly visible it is especially at night, so they often prey on weak and sickly individuals. There have been many sightings in fact of prey resigning to their fate and allowing the Shadowreapers to eat them. This combined with their apperance and unusual powers is the basis for why they are know as the "Horse of Death".

(cont.)
They are solitary by nature (helps to limit how visible they are) but they will sometime travel in small packs when they are young.
They don't often associate with other creatures due to their nature and diet.
They typically live in heavily wooded forest areas (they don't do well in big open areas) but are capable of surviving in a variety of habitats.
They are very territorial but they don't fight each other very often. Instead territorial battles consist of each Shadowreaper creating big and complex weapons with their shadowy manes as it is a good and accurate display of strength. The bigger and more complex the better. However, if they are similar in strength fights will break out in order to determine the stronger of the two.
This also applies to mating rituals but not in the way you'd initially think. Females and Males will pair off with those of similar strength.

That's all I've got for now.
Glad you like him
And remember just enjoy and have fun with him :)