How wouid the story have changed if the one surviving animal had been the hyena?
You would need to find out about the hyena to know their habits in order to answer this question fully. Then you could apply this information to the Life of Pi and create your story. I'll give you a few facts to suggest what might have been if the hyena had survived.
The hyena has one of the strongest jaws in the animal kingdom and fears only big cats, such as the lion or the tiger. Spotted hyenas are highly intelligent predators. They hunt in the same manner as dogs, chasing their prey until the animal gets tired. They tend to eat their prey while it's still alive. They have been known to kill humans.
I don't think the author would have ever chosen the hyena as the survivor because most people view them as dirty, disgusting animals. We view the scariest kinds of predators to be the lions and tigers. Remember The Wizard of Oz? "Lions and tigers and bears! Oh my!"
bmadnick's answer can also be observed and referenced in the novel. If you refer to the end of chapter 43, Pi provides extensive information about the appearance, habits, and deameanor of a hyena. He describes the cruel fashion in which this creature hunts and disembowels its prey while occassionally committing "accidental canibalism". The hyena's foul "catholicity of taste" showcases its innate desire to quell its hunger. Pi states, "it's an open question as to what hyenas won't eat." Had the hyena survived, "things (might have) ended in typical hyena fashion".