A person or thing that has seen long service or has lived through many hardships and can be relied on: “That teacher is a real war horse; he has seen the dismissal of ten different principals.”
Also, old war horse. A dependable, frequently performed attraction, as in The opera company is doing nothing but old war horses this season, like, Aïda and La Bohème. This term originated in the mid-1600s for a military charger that had been through many battles. In the 1800s it began to be used for human veterans, and in the mid-1900s for popular productions, especially of musical works.