Zack becomes Stan's personal trainer in "The American Dad After School Special" when he believes he is too fat. Zack is a tanned, blonde guy who wears wife-beaters and trucker hats. Zack is a relentless taskmaster but it is strongly hinted that Zack exists only in Stan's imagination. None of the family sees Zack but Stan and when Stan decides to get help and quit pushing himself to be thin at all costs, Zack fades away.
Zack was voiced by David Herman.