Shadowmere is a horse in the game Elder Scrolls IV: Oblivion. You acquire it during a Dark Brotherhood Quest from your master and keep it for the rest of the game. It is THE fastest horse in the entire world of Oblivion. It is also a "hero" character, meaning you can't kill the thing, only knock it out.
-Holy crap! Before it took 12 hours to get from Chorrol to Cheydinnhal. But with Shadowmere, it now takes a mere 6!