I think Isobelle and Thunder was a good example of how you can turn one possible horse scene into the whole story line.
I agree, it was the best bestiality storyline in the whole game. And it's a pity that, almost two years after it, there's still no continuation with the ranch...
I'm still amazed, considering the fantastic-fantasy plot, that Isobelle is not given some sort of magical potion or whatever to give birth to Thunder's offsprings...