Shadowmere (The Elder Scrolls IV: Oblivion)
First mentioned by Knight Captain Kerr
Shadowmere is a horse that once belonged to Lucien Lachance of the Dark Brotherhood. It's passed on to the Hero of Kvatch after completion of the quest "The Purification." Not only is Shadowmere the fastest horse available in the game, but she can also climb inclines that the player cannot. In addition, she is very aggressive and has high health, which makes her a valuable ally in combat. As she's an essential character in the game, she can't be killed. Instead, if her health falls to zero, she falls unconscious, reviving a short time later. There was even a storage exploit that allowed her to carry infinite items, as long as you knocked her out first to access them.