The skeleton is based on fake, or real skeleton models that one often can find in science classes or doctoral work. It's body shape gives viewers a possible insight as to what the Guppies also look like on the inside (if not for the song itself.)
While the gender is unknown, its possible the skeleton is/was older then the guppies. Given how long/tall it is. Its bones are pure white and it lacks any sort of nose holes. It's normally seen smiling, and has big holes for eyes.
It also has a tail fin like the guppies do.
- Despite not being afraid of this skeleton. When Gil first saw Molly's right after the song he was very shocked and terrified and ended up running away after someone put the fake skeleton behind him.