I was just going to add a picture of Laika

on the way I stumbled upon Some Unknown Pages of the Living Organisms' First Orbital Flight a short writing presented by D C Malashenkov about his recollections of the Sputnik 2 project at 2002 Second World Space Congress.

1954 during the meeting in the Academy of Sciences of USSR, the basic research problems conducted by means of artificial satellites of the Earth were determined.

...

The successful launch of the Earth's first artificial satellite [Sputnik 1] in October 4, 1957 has made stunning influence on all the world. To continue this success, N. Khrushzev in October 10 1957 stated to launch the second satellite with an animal onboard till November 7 (40 years of October Revolution). The level of complexity of forthcoming tasks was much higher. Tightness of a cabin and the systems of life-support of the satellite should provide considerably large duration of flight under previous dimensions and power consumption. The research equipment should also ensure long uninterrupted registration of the scientific data and their transfer on ground stations. At last, the realization of additional training and special preparation of dogs was required.

In 1954 they make a To Do List. By 1957 it is a Done List. Then it is decided to do Sputnik 2 (with Laika) 1 month later. While they had been doing work to build up to sending animals up, it was not expected so soon. They had to do all the training, testing, and engineering in 1 month!

You can read what they did in the link. Apparently this is considered definitive regarding the manner of her death, as previously there were different accounts.