I’m more of an artist than a writer, but I think if I was in your situation I would publish it on a major platform like Tapas, Wattpad, or AO3, maybe even tumblr, and use other sites to advertise your work and generate traffic like Twitter. I still see a huge feral audience on DeviantArt too. I think a lot of it is just going to be advertising your work. I wish you the absolute best.
Edit: Tapas has a pretty good forum you can advertise on if you publish on their site
I have an animal fiction story that I’m planning to write, but I dont know where to post original stories like that. I know you can post original works on ao3 and watt pad but most people go to those for fanfics so I’m not sure how many people would be willing to read it on those sites. I was wondering where the best place to post this kind of story would be?
Also I am willing to post on multiple sites. I’ll probably post it to tumblr, but I’d like some other places too.
It depends. Readers are something you have to cultivate, so it tends not to matter what platform you use. I'd recommend either FictionPress or Wattpad. I've not had much luck with the latter, but you are familiar with it, so I won't go into detail. The former is the original sorry version of FanFiction.net. It takes time to get readers, and you may face hash feedback, but it would be my top choice. I don't post my original works anymore—outside a forum meant for feedback—but those are my recommendations from when I did.
I think just posting to everywhere and anywhere. I typically post to Ao3 just because of its tag system. But I also post finished works to toyhouse for character reasons. And as stated above, it takes awhile to gain a following, but if you keep at it you'll get some people following! I recommend to try to shoot for a regular posting time. So once a week, once a month, etc etc. Just so people know when you'll post.