Need a spot to hold all that extra gear? Use your horse!

First you must complete the Dark Brotherhood quests up until you receive Shadowmare as a reward. Shadowmare is special in that he cannot be killed (only knocked unconscious). Knock Shadowmare unconscious (you don’t need to be quick about it, he won’t fight back). As soon as he drops, you are given the option to access his body via the spacebar. Open him up and dump your extra junk into him (open him then go to your inventory button and single click items to move them over). Shadowmare will pop back up after a few seconds, and you’ll both be good to go.

Whenever you need to access your gear, simply knock him out again. I’ve used this trick for a while, and it appears that Shadowmare has no weight limit, and items remain forever. This trick won’t work on normal horses because they die (and ~ resurrect clears their inventory).

Sometimes after Shadowmare wakes up, he begins to slowly walk away, to stop this, jump on him and quick transport to a location; this fixes him. Also, be careful not to get onto a regular horse, this messes up your “tie” to Shadowmare, and is a good way to accidentally lose him (not sure where he defaults to).

shadowmere defaults back to the fort where you first get him, in case anyone is wondering.  I've lost him quite a bit over the game.  I also figured out that trick myself and have been using shadowmere whenever I become over encumbered.  He contains a full set of mithril and glass armor, a large collection of random dwarven weapons and armor, and much much more, all in his tummy.  He also has my wajabbit or whatever it's called.

isn't there a mod out there to make shadowmere a mule?

By the way, who is the best to kill to get in to the Dark Brotherhood