Question On Tying Your Own Flies (Fly Fishing Experts Please Read)?

[Raiders- AFC West Champs]

Can you tie a fly any way you want? or does it have to look exactly like a fly. Because i have been learning about tying flies and have made a very pretty attractive one. And i doubt any fly would look like that but would a fish bite it. so my question is can you make a custom fly or do you have to tie flys that actually exist.

Thanks For Reading And Take It Easy On Me, Im A Beginner.