He could be nipping his own fins (bettas with large, heavy finnage are more prone to this behavior). I have at least 2 fin biters on my hands now, a VT and I suspect my delta tail too although at first I thought it was from flaring too much at his neighbors. I have 7 Amanos in with my large finned Halfmoon Double Tail and they just avoid him completely. As soon as they touch him with their feelers they swim away. Plus, I'm not sure they actually can nip their fins because they aren't strong enough.
10 gallon planted - HMDT male betta, 4 peppered corys, assassin snails, Amano shrimp
10 gallon divided bare bottom - HMPK male, 2 VT males
29 gallon sorority planted [journal] - 6 females, 4 bronze corys, trumpet & nerite snails, 3 Amano shrimp