bordom? i dont remember, another member here had the same problem but i cant remember what ended up being the reason behind it make sure u treat the water with salt to keep him from getting infected.
I had this problem. My bettas were constantly flaring at eachother (they're bowls are next to eachother), and so one of them chewed his own fins. I moved him to a location where he couldn't see the other betta anymore, and he stopped chewing.
1 of 2 reasons. Either it is wanting to fight another male, and it's tail is the only thing that looks like a male betta ..or it is getting the first stage of finrot, and wants to get rid of the disease. In the wild, if a betta gets finrot, it's life is over. There is no meds to heal it. they will chew off the infected part, so it stops spreading, and then the tail, grows back.