I was not a fan of the Arvak model (it looks so uncomfortable). Having destroyed the Dark Brotherhood Sanctuary led by Astrid, I found in my character a significant lack of Shadowmere.

Ipso, facto, meeny moe magico! Arvak with the Shadowmere model!

This mod does very little, really, and hardly seems worthy of a Nexus release. But, hey, I wanted it bad enough and could find nothing that would do the same, anywhere. So, for the CK-challenged, I've uploaded the quick fix here.

Obviously this REQUIRES Dawnguard, as I've done nothing to change the Arvak summon spell itself. I've merely changed him to utilize the Shadowmere model.

- Change Log -
Version 2.0: Brought Arvak's behaviors back in line with what was intended (less aggressive). In addition, there are now two different options, one with and one without the Dark Brotherhood saddle.
Version 1.0: My original attempt unfortunately caused Arvak to inherit a few of Shadowmere's behaviors, as well as the distinctive saddle. This original file is retained under old files, just in case that sounds appealing. :P

- Installation Instructions -
Manual: Copy the .esp file into your Skyrim /Data/ directory. Done!

- Uninstall Instructions -
Manual: Delete said .esp file from the aforementioned Skyrim /Data/ directory. Done!