Berylglow Nix-Ox Steed

"Yes, Gemmy here glows. The Hlaalu of Balmora fed the first of its kind a rare mushroom, causing a reaction in its belly. The glow passed to its young. They're fast but feisty. Don't say I didn't warn you."—Cidius Sosia, Necrom Stablemaster

Source: in-game description.