The mod that modifies the hotbar texture like that is SquidMod... It's a really old mod which I can't seem to find on any of the mod sites, but I imagine with a TOC update it should still work.
*Digs through 2 feet of old-addon-rubbish at bottom of hard drive*
Here it is! Have fun!
As I remember, it includes a few fun racial graphics (i.e. undead, night elf, etc.) as well as a few other random ones. To pick one in-game type /squid.