Jack the Yak

Jack the pac yak was the fittest in the stack.
He could thwack a backpack to a person riding horseback.
He then had a anxiety attack, witch lead to a heart attack.
He now could only eat crackerjacks with a lumberjack.
He drove his cadillac into a wrack of knapsacks.
He was overwhelmed by the negative feedback.
He then hit the sack thinking about lilac, his jet black yak.