Many people have guessed it, and now we'll confirm it: the creature being hit by the lightning in the Onslaught version of Shock is in fact the lovable Psychatog.
Ed Beard's original Shock art was moved to another card in the set, and when asked to do another version, he decided to have a little fun and show Psychatog—by that time already the scourge of the tournament scene—getting fried.
Of course, Shocking a Psychatog is not good for much in game terms, but it's nice to see the little sucker suffer.