As you know koi fish is thought to be a symbol of success in Japan.
An ancient tale says there is a vigorous upstream somewhere on the Yellow River. The waterfall at the end of the upstream is called the Ryumon-Gate (literal translation is Dragon Gate).
Many koi gather around the basin of the Ryumon-Gate. If a koi swims the waterfall up it’ll become a dragon and fly away.
Bakegoi (Dragon fish)
The koi gets spiky in progress. Single/double horns grow and the fins get bigger.
The fins turn into wings.
Legs grow. It's called Ouryu in this phase.
The tail gets longer.
Finally it swam the waterfall up and turned into a dragon.
by Tattooist Ryugen