I concur with what everyone else in this thread is saying. Why bother telling someone something they don't need to know? The fact that you're making this thread, though, seems to show that, for whatever reason, you do have a motivation to tell this person. I would heavily, heavily, heavily interrogate that motivation. You should be able to tell us (and more importantly, yourself) exactly why you want to tell someone you're a zoophile. If you can't do that with confidence, you should not tell them yet.
If you decide to ignore what everyone in this thread is telling you, this is how I would approach it. Note, I have never told anyone in real life, so this is purely hypothetical. One way or another, try to broach a conversation ABOUT zoophilia without saying "I am a zoophile". Say you saw people talking about it on twitter or the news or something. Zoophilia is a taboo topic, so you can expect this person to kneejerk react negatively. I mean, I'm a zoophile and if a friend brought up zoophilia to me in person, I would also instinctively react negatively. It's an instinctive tendency to seek social approval. In response to this, you can say something like "that's what I thought at first, too, but the arguments I saw said X, Y, and Z and it made me reconsider, what do you think?" This would be the turning point, where you're asking your friend to actually engage with the morality of zoophilia. They will react either negatively or positively. If they react positively, you can keep discussing it and work your way up to saying something like "I'm glad we agree on the ethics of zoophilia, the truth is I am actually a zoophile myself." Most importantly, if they react negatively, you haven't committed yourself to anything yet. You have kept an off-ramp for yourself. You can just say "oh, yeah, that makes sense. I thought what I saw was convincing at first, but I think you're right. Zoophilia is wrong after all." And then never bring it up again.
If you're really committed to doing it, I hope the above strategy is helpful. Like I said, it's purely hypothetical, but I can't think of any better approaches. Despite writing all this, I don't think you should do it. Unless this is a romantic partner, the risks are high and the benefits are small. If you're looking for understanding and validation, it's better to seek it from places like this to risk ending friendships (or worse) by discussing it with IRL friends.