It's damaged, probably from being bitten by a tank mate at some point. It's good that it isn't changing. Keep an eye on it, looking for growing shriveled areas or open wounds. If you don't see that, then the dent may just stay like that going forward, or if circulation is cut off, the end may fall off. That's OK as long as it's healed over and doesn't get infected. I don't see signs of tail rot or a major issue in that photo. Just keep an eye on it.