The first person I told was my best friend while we were in college together and, like some other people have said, I didn't exactly come right out about it. I say best friends, we grew up together, our parents were friends before we were born, it felt like something I needed to tell him. In addition to wanting someone, anyone, I could talk to, since he was always the one person I could talk to about anything. I already knew we were both into furry porn, which seemed like a good first step, and I had known I was a zoo for around five years by then. To be honest I had it easy in regards to telling him. we were out on a weekend long camping trip which we did pretty often, and I suggested we play "would you hit it?" (which is where you each take turns naming a person or fictional character and the other person says whether or not they would have sex with them, for anyone who doesn't know). My plan had been to name off some popular furry characters, then ferals, then actual animals. To my surprise it was only like his third turn when he said "What about a dog?" to which I enthusiastically answered "hell yes". And it went on from there, we talked a little about it every once in a while and ogled Bad Dragon's toys and the like. A couple of years later he even kept an eye out for me while I banged one of my parents' mares, though he was never into horses himself (his loss).
The only other person I've ever told was a girl I met in college. We were friends for a couple of years and decided to start dating, and before we made it official I felt like it was important that she know everything about me and my sexuality. I told her there was something I needed to tell her about my sexuality and she confidently said "Don't worry about it, there are only three things that are a 100% relationship ender for me: Children, family members, and pets". Of course at that point she knew there was something I wanted to tell her, so I couldn't very well back out or she would assume the worst so I just had to tell her. And of course she was upset and she had questions but in the end we did end up dating, with the understanding that I wouldn't be involved with any animals while we were together. And then we got married, and I do love her, but the zoophilia is something we should have come to an understanding about beforehand. It would have been much easier than digging it back up now.
Aside from that there are at least three more people who know, all of whom found out through my wife unfortunately. Two of them read her text messages with me, not really her fault and an invasions of her privacy as well as mine but one of those people is a genuine asshole and I would have been much happier without them knowing. The third I'm pretty sure my wife may have drunkenly told at a party we hosted, but honestly if anybody was going to be accepting of zoophilia in our friend group it would have been that person so I don't really have a problem with it other than the fact that my wife told someone.
Sorry, this got really long and rambly, but long story short if you're in a committed relationship with someone you should probably tell them because the longer you wait the harder it will get. Telling anyone else is a personal choice that only you can make, and honestly I don't feel the need to tell anyone but my closest friends (I have a few other very close friends I've made through the years who I will tell at some point, when the time is right). I don't think I'll ever tell anyone in my family. To be honest I don't think they need to know, and it just has the chance of causing enormous problems with no real chance of benefitting anyone (at least in my situation).