rectum would mean something is straight but in case of the rectum it has some tight bends which internally are represented as rectal valves (transverse folds)

Thank you dear for your answer.

Yes in case quadripeds like dogs Rectum is Straight and not Curved as in humans. In humans Rectum has curvatures in Saggital ( sacral and Perineal flexures)  and Coronal planes ( Upper right,middle left and lower right). so the term rectum is a misnomer in case of human beings.