Fighters which became obsolete at the dawn of WWII
The Heinkel He51 entered service in 1933 and left active service as a fighter in 1938, but remained in use by the Spanish Air Force until 1952.
The Hawker Fury entered service in 1931. And her frontline service with the RAF ended in 1939, when she was replaced by the Gloster Gladiator(!) and the Hawker Hurricane. In 1940 the Fury was still in use by several other air forces and were in front-line use by the South African Air Force against the Italians in East Africa in 1941
The all metal Dewoitine D500-D510 enter service in the French l'Armee de l'Air in 1935. The aircraft was replaced by the more capable Morane-Saulnier MS406 in 1939. Although the Dewoitne was sold to several countries. It didn't play any significant role after 1939.
The metal Boeing P-26 Peashooter enter service with the USAAC in december 1933 and were the front line fighter of the USAAC until 1938 when the P-35 and the P-36 started to replace them. The last user of the Peashooter was the Fuerza Aérea Guatemalteca (Guatemalan Air Force), where the final pair remained in military service until 1956, when they were replaced by P-51 Mustangs
Although a beautiful aircraft, but Cees Donker's Fokker D.16 doesn't really fit in this row as the D.16 had already been replaced by the Fokker D.17, which was already replaced by the Fokker D.21 in 1938.
Cheers,
Huub