This is answer 1A in "BizQuiz: Working with overeager horse owners."
1A. Wrong. You know more than just about any equine client, but clients who take the time to research their horse's possible medical issues and bring that information to your attention mean well.
The clients who share what they know often bring Internet research and reading material. Don't discount their opinion or sources of information. Instead, look at the materials, and acknowledge their efforts.
These clients usually mean well; they're not trying to undermine your authority or toot their own Internet-search-savvy horn. So while you may feel that they don't trust you, they're just thinking about their animal and its care. They're looking for the best answers.