Its a question of what you're willing to sacrifice. If I have been with someone for years, it might not be worth it to bring it up. If they would leave you for voicing an interest in something very taboo, then maybe they arent the right person, or they are, and your taboo interest isnt worth losing them.
My point is the issue is very simple, Is it worth losing them?
Its just not easy. Its simple, but not easy.
Take some time to reflect about why you're interested in it. Why do you feel the urge? Maybe its a certain aspect of primal instinct. You could get that from your partner, or an animal. There could be a solution that doesnt involve an actual act of beastiality. If its the act of knotting there are toys or special penis sheaths (if your partner is male) that they could wear to simulate a canine penis.
For me personally I learned that being with someone who is also interested in beastiality is a deal breaker for me. Is it a deal breaker for you? Then ask them. Otherwise, find another way to scratch that itch, with your partner.
Whatever you do, do not let it just sit. Lack of communication kills relationships. I assume you know that though, ten years of experience and all.