What was happening to Nellie, and why did Billy have to kill her to take out the colt?

Nellie,
Jody's mare in , was having trouble giving birth because the colt was
turned the wrong way inside her. Billy knew that she would trying to give birth and that the
colt would probably die as well. By killing the mare, he does the only humane thing he can think
of to do; rather than let both horses die in agony, he kills the mare, cuts her open, and
removes the colt.

Nowadays, we would probably call a...