I had a male halfmoon betta that I was keeping temporarily in a floating breeder mesh box in my bigger tank. Completely my fault for not securing it, but somehow during the night it got tipped/snagged underwater, opening it up to the tank. I had 5 tiger barbs at the time, and they absolutely shredded the poor betta. The next morning, he was floating on his side at the top, every single fin he had was ripped off. I thought he was dead but noticed his gills breath once every three or four seconds. I moved him into a planted 10g, and kept him inside the mesh box so he was near the surface. He lay there for about a week, not eating, barely breathing- just drifting on his side in the current.
Suddenly, one day, he was wriggling through the water (finless!), had a healthy apatite, and breathing properly! It's been about 1.5 years since, his fins are all grown back, though they're a bit shriveled and discolored, and he still wriggles weirdly when he swims. But he seems very happy, curious, and he's even mated a few times with my little red female.