Hiya, Emma! Welcome to planDisney!
It sounds like you have a fun Walt Disney World adventure in the works. Let’s take a look at the Walt Disney World dress guidelines and park rules to make sure you’re all set to enjoy the Vacation Kingdom of the World.
You can find the Walt Disney World Resort attire policy
here, but I find
this FAQ page to be a little easier to follow. According to the dress guidelines, costumes may not be worn by guests older than 14 years old except at
specific separately ticketed events like Mickey's Not-So-Scary Halloween Party, Mickey’s Very Merry Christmas Party, and Disney Jollywood Nights. If you’re just wearing a tail, then it would probably be considered an accessory and not a full costume, and you should be fine.
Keep in mind, Disney Cast Members have the final say on what is or isn’t appropriate attire, including attire that could detract from the experience of other guests. If you’re worried that your Na’vi tail doesn’t fit the dress guidelines, then you could check with a Cast Member at the park entrance. You could also call the Non-Reservation Support line ahead of your visit.
I hope you have a wonderful visit to Walt Disney World, Emma. Don’t hesitate to send any other questions you have to your pals at planDisney. We’re always happy to help!
Magical travels!
Matt