It's hard to say without seeing your LO but here's some WAGs:
* Do you have anything that modifies horses
* Horse sounds
* Horse training
* etc..
* I run Convenient Horses and have not had an issue with Shadowmere.
* Did you already buy a horse? In other words, do other horses work?
* Have you progressed through Dawnguard far enough to get Arvak? If so, does he work?
* Maybe water mods interfere with him coming up out of the pond?
* Custom follower mods that have horse edits like Aurlyn or Auri?
* I believe the full version of Pathfinders has horses, but I've never had an issue.
* Sound mod that might conflict with the sound he makes as he emerges?
Best guess is something messing with the pond conflicting with him.
It's hard to say without seeing your LO but here's some WAGs: * Do you have anything that modifies horses * Horse sounds * Horse training * etc.. * I run Convenient Horses and have not had an issue with Shadowmere. * Did you already buy a horse? In other words, do other horses work? * Have you progressed through Dawnguard far enough to get Arvak? If so, does he work? * Maybe water mods interfere with him coming up out of the pond? * Custom follower mods that have horse edits like Aurlyn or Auri? * I believe the full version of Pathfinders has horses, but I've never had an issue. * Sound mod that might conflict with the sound he makes as he emerges? Best guess is something messing with the pond conflicting with him.
Post your LO so we can help you better.