I got a mystery snail once, and my betta attacked it twice within a couple of minutes, including biting off one of its eyestalks.
So I took the snail out. Eventually, he was in a 10 gallon divided tank that developed an algae problem. I was afraid to put a nerite on his side because of the way he acted with the mystery snail, but eventually I put a nerite in with him, and he totally ignores it. Nerites hardly protrude from their shells at all, so the betta basically just sees a gliding "rock" from it's POV, rather than this weird creature emerging from a shell and acting like it owns his tank. haha.