1. He's using Parrot (which now has about 20 animation styles, one of which probably looks like that screenshot) and set the font to the one he's using (I don't know what the name of it is).
2. He's using Blizzard's Floating Combat Text, and put renamed font files in World of Warcraft\Fonts to override the default fonts, and is using a modified version of ClearFont2 to use yet another font for everything BUT the Blizzard FCT.
Given those two options I'd say #1 is vastly more likely. :P
My money's on FCT with the modified font, actually. If you look carefully you can see the font is suffering the same pixelation problem commonly seen with FCT - and it seems to be "over" the mob in the way FCT can do that nothing else seems to quite get right.