More commonly know as Giles or Gi.
This is a picture of what he looks like in what he thinks of as his "birth-form" as opposed to his "true form" (that is, all human, no horse) or his "horse-form." The fact that he's the most comfortable in this form makes him feel vaguely guilty.
NOTE: Due to recent website funkiness, you have to select the "original image" option to view it.