You will not forget Gaebul easily, It resembles the penis with its colour, shape and size. In English it is called "penis fish" and its original name refers to male genitalia. Science applies the Latin name 'Urechis unicinctus'. It is a marine worm that is found in Russian, Chinese and Japanese coastal waters at a depth of about 1700 meters. It is easiest to catch in the East Sea. On the Korean shores it is usually located in more shallow depths of around 50 metres.
If you find it, go to one of the Korean fish markets. This sea worm is basically fresh. Small pieces are served while still alive. Larger portions are served with noodles, sesame oil and salt.
https://www.youtube.com/watch?v=7KqlAMqkxt0
https://en.wikipedia.org/wiki/Urechis_unicinctus