I don't know if this is normal, but s/he attacks enemies that haven't even engaged with me. So I'll get off the horse, and Shadowmere will take off to fight someone, and then I'll have to go and find him/her.

Today I got off the horse after climbing up a mountain, then it took off downhill, to join in some fight below, and I guess someone cast Turn undead because they all took off chasing that skeleton and I no longer had a horse.

Is this normal and is there a way to disable it?

Shadowmere is a crazy horse,sometimes I wish I had never taken the quest that far and not had to deal with it,I just got to the point when the darn animal runs off I just go on about my business....he/she always shows up later on!

Ahh, looks like gamesas forgot to ensure that Shadowmere had companion AI.

This is normal for a non-companion NPC or Critter. If it's of a hostile faction and it notices the enemy, it's gonna do it's best to kill it, unless its AI dictates otherwise.

Its still a problem in Skyrim hee, and there's probably no way to disable it. Unless you use mods. So many people have complained about crazy horses. At least shadowmere's immortal, and i think she'll come back when you fast travel or something.

No, she didn't. I tried to fast travel, and wound up alone. I tried a mod called Dude, where's my horse?, which works until she dies, and then it's like she's no longer your horse and returns to Farragut.. eventually.

There is a way to not get shadowmere at all. If you go straight to Lucien through the hollowed out tree after you "purified" the sanctuary the horse shouldn't follow you so long as you don't get on him.

Arg damn horse. Parked her far away so she wouldn't get into the fight.

Used demoralize on one tough enemy so I could focus on the other one, it ran straight for Shadowmere, who fought it, died, went tumbling down the mountainside comically, and upon resurrection walked away.

I've been using
player.moveto 1e002122
to teleport me to her after that. But the first 2 digits of that ID changes each time you add mods.

I think the most irritating thing about Shadowmere was its endless horse-noise-making when it fights something.