Okay, so we all know that shadowmere was present during both the events of oblivion and skyrim, making him at least 200 years old, probably older, as he was owned by lucien lachance before he awarded him to the hero of kvatch. I'm no horse expert, but im pretty sure they dont usually live to be 200 years old. So what exactly is he?

We also know that it is possible that he is genderless as he is referred both as "he" and "she" in both games, if this is the case then what exactly is he/she? Could it be that he is some sort of Daedra? Taking his genderless and seemingly immortal characteristics into account this would make sense. But if he is then what prince does he belong to? Could shadowmere be conclusive evidence that the night mother is mephala? Could that pit he rises from be a portal to oblivion?

Or is shadowmere something else?

All i know is that he is no ordinary horse. So what is he?

If it is the product of a Daedric lord, I'd wager it is Namira since she is associated with repulsive things (and thus undead things). Is there a specific Daedra lord associated with the undead (not vampires, but undead, like zombies, etc)?

That, or it is possible it is the product of necromancy, perhaps the first Speaker of the Dark Brotherhood gave rise to it.

I'd guess that its more along the lines of the latter.

I was thinking that shadowmere was more likely a daedroth taking the form of a horse, than it actually being a horse, dead or not. Which would explain its changing gender and whatever that pit is found near being a portal to oblivion of sorts.

And no i dont think there is a daedric prince directly associated with the undead. I think the closest one would be molag bal or clavicius vile even, considering his dealings with n'gasta and how he turned a village of kothringi into the undead to "save" them from a disease.

A Daedric being. Not a Deadra. But a being of the Void. Remember the Quest Paradise in Main Quest Oblivion we met Ascended Mortals who could not die. They will always reincarnate/reborn as they are, forever trapped in that state of being... like Daedras themselves. In a way... those mortals are Daedric.

We have another example of beings of the Void like. Haskill and Dyus. Both beings cannot die. Dyus mortality has been taken away by his Prince. Haskill is a mystery but the way the two are compared and refer each other as colleagues, it could be said he also has a similar case except Sheogorath was involved instead.

Though who Shadowmere really belongs... we don't know. There's nothing in the lore that contradicts Mephala as Night Mother, actually it is very fitting... but it's speculation... heavily implied speculation with good groundings. I subscribed to it because when we look at Sithis, he is depicted with a hole in his heart. And there is the fact the whole Dark Brotherhood origins connects to the Morag Tong, Dunmer culture that's taken from the Aldmeri stock which revolves around Psijic endeavour.

What is guaranteed what Shadowmere is that she/he is arguably a being of the Void (not just some necromantic creature). But whether she is a Daedra or something else entirely remains a mystery.

Though the lore surrounding the issue has grown a little fuzzy since then, Mephala was the head of the Dark Brotherhood back in Daggerfall and would be occasionally refered to as the Night Mother.

With the relations between Mephala and the Night Mother, it seems fitting that Shadowmere changes from female to male, given Mephala's frequently changing gender.