Working animal (9)
(Other definitions for carthorse that I've seen before include "Beast of burden" , "Equine" , "has pulling power" , "large drawer" .)
(Other definitions for guide dog that I've seen before include "Canine for the visually-impaired" , "Seeing eye" , "Blind person's aid" , "Aid for the visually impaired" , "Blind person's helper" .)