Betta splendens – known as simply the Betta, or Siamese fighting fish.
This is a fish that has been kept by aquarists for decades, offered in a huge range of forms. Males are the most commonly offered, with them not tolerating other males except for very specific scenarios. It is absolutely recommended against mixing males in all but the largest of tanks, absolutely filled with plants. It will most likely always end up with one or more sustaining serious damage or death.