Cosmonaut dogs Mishka and Chizhik. 1951.

The dog-cosmonauts Mishka and Chizhik flew into space twice. The first flight on August 15, 1951, was successful. The second flight took place two weeks later, on August 28, 1951. The launch and landing were normal, but the dogs were found dead. The cabin depressurized, and the dogs died from lack of oxygen.