I think there's nothing wrong in trying to align one's work to one's ethics. The issue is not Lisa's veganism, but Lisa's writing. She could have, and should have, dropped a line to explain what happened.
The thing that bothers me is not Rousseau's absence. The horse not being physically present in S3 makes sense. His function in the story was concluded by the end of S2. Sure, it could have been reintroduced in S3, but what for? I'm not surprised it was written off, regardless of the reason behind that.
What I find totally unrealistic is Sara never asking August what happened to the horse. Or, August could have mentioned it talking about the assets he had to sell for the settlement. Or he could have reassured Sara that the horse was well and was being taken care of, in one of the scenes where he tries to gain her sympathy back. Or whatever.
Personally, I'm not disappointed by Lisa's decision being dictated by her moral values, I respect that. But Rousseau was not "lost into oblivion" because Lisa wanted the show to be vegan, it was because she has this habit of dropping elements of the story and then handling the script as if they never existed in the first place. Unfortunately, there's plenty of examples in the show, besides Rousseau.
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