Its just a gnome DK on a snowy gryphon. It doesn't look nearly as cool on the other mounts.
What bothered me most is how my MOUNT got stung when he wasn't even there during the battle

*edit* I just went back to check. You get a pink glow from Marshlight Bleeder wasps, and a green one from fenglow stingers. Blacksting gives a black "glow"... which looks pretty much the same on all mounts.
Interestingly only the armour of the celestial steed changes, as well as the stars in its body... the rest is still glowing!