Yep, the goblins in the CN:H / AYAD world aren't exactly as they seem. They choose a form they enjoy and tend to stick with it. This goes for most of the Fey. The stronger the Fey, the more forms they can control. Lesser Fey, like Greer, tend to only have one illusion form. Greater Fey, such as Gavin, may have a handful of forms. Truly powerful Fey, like Puck... well, no one knows just how many forms he has, to be honest.
So what does a Fey's true form look like? No one other than that Fey knows. It's a secret they're not willing to share.