Laika
Laika died within hours from overheating.  Her body temperature got way too hot for her to survive. The heat in her spacecraft had quickly risen to 40 degrees Celsius (104 degrees Fahrenheit). The true cause and time of her death were hidden from people until 2002.  Instead, it was reported that she died when her oxygen ran out on day six. Another untruth, as the Soviet government first said, was that she was euthanised (put to sleep) before her oxygen ran out.