You can take your sim into CAS through the mirror or dresser. Once there, set up a bear or knight costume as one of their everyday outfits, 3 or 4. On bear night, have your sim change into the outfit and send them to the bar.
Sometimes people call my sim for alien's night and I'm pretty sure you can't even disguise as an alien. I always think of my sim declining and saying: ''Do I look like an alien to you!?'' or ''What are you trying to say here!?''. Hahah.
Comments
This is a list of the special bar nights and the days/theme in case it may help you.
http://simsvip.com/sims-4-get-together-guide/#BarNights
http://forums.thesims.com/en_US/discussion/817478/hello-come-and-introduce-yourself
http://tinyurl.com/OneRoomOneWeek
http://tinyurl.com/rosemow
My Showcase thread https://forums.thesims.com/en_US/discussion/948861/rosemow-s-rooms-showcase
You're welcome I hope that your sims will enjoy attending the special bar nights,
http://forums.thesims.com/en_US/discussion/817478/hello-come-and-introduce-yourself
http://tinyurl.com/OneRoomOneWeek
http://tinyurl.com/rosemow
My Showcase thread https://forums.thesims.com/en_US/discussion/948861/rosemow-s-rooms-showcase