Maybe souls reincarnate anywhere into anything.
Or maybe a soul only reincarnates into related beings. In the story of the The Goat Who Saved the Priest, the goat itself had been a priest in a past life. There was a relationship to its actions in past lives and the form it took. I've heard versions of that story that suggest the goat had been the very priest it 'saves' (because souls can reincarnate in the past or future?).
Perhaps if we had intergalactic flight and our actions impacted creatures on other planets, we'd reincarnate into those creatures. Then again, perhaps we already do.
If you like livermush and you're ever near D.C./Maryland, try scrapple, which is almost the same thing.