1. A veteran of some activity or profession (not necessarily the military) who has a wealth of experience going through various trials and tribulations. I know she's a little eccentric, but Janet is an old warhorse in this firm—she's been through more trying court cases than anyone else here. The president's pick for Secretary of State is an old war horse of international trade.
2. A performance or dramatic work, especially a musical, that is widely known and reliably popular due to how long or frequently it has been performed. Announcement that the old war horse was being revived for a run on Broadway sent a collective cheer across the nation. The theater has a few old warhorses that it brings out once or twice a year that it knows will always fill seats.
a performance piece that is performed often. (*Typically: be ~; become ~; perform ~; play ~.) The symphony orchestra played a few old warhorses and then some ghastly contemporary stuff that will never again see the light of day.