Apparently some mad engineers called G.C. Johnson and J.W. Wilkey actually thought of sending pigs to space instead of monkeys or dogs. It was 1963. Imagine that. PIGS IN SPACE.
It actually makes sense, since pigs share quite a few common physical points with humans. Pigs' organs and layer structure under the skin are very similar. The heart is so similar that pig valves can be used in human hearts after a four-week treatment. The liver and the gall bladder look almost identical, and the digestive system is also extremely similar, both in how it works and how it is structured.