When I was 8 years old, I believed that mermaids existed. My mother told me that I had cried when I saw a little mermaid figure go up in smoke.
However, as adults, we were too busy living a social life and have forgotten the innocence of childhood. Time passed and the child became an adult who have outgrew beliefs in magic and mermaids alike. Amidst social commitments and the pursuit of wealth, the belief of mermaids were long forgotten ; signifying an end to the childhood innocence - symbolised by the tail (and tale) of a mermaid.
As a space businessman who conquers, maintains and sells stars, Frieza was busy with business dealings and combat activities in his growing years which I believe had led to him forgetting about his innocent childhood days. This resulted in him becoming a villain over the years, before he knew it. I had thought about how there was a time in Frieza’s younger days where even he believed that mermaids existed.
In his pursuit for success and control over the universe, Frieza, the main antagonist in Dragon Ball, forgets all about the mermaid and became a supervillain. On the other hand, his rivals Son Goku and Vegeta were very strong as they were free spirited, and only had to pursue strength. Clashing with them forced Frieza to train rigorously, growing and harnessing the power in his tail (as seen in the stories that ensued). Training to be the strongest while trying to control the universe had began to take its toll on him.
This version of Frieza was torn between the pursuit of strength and full control, to the point he had wished to return to his innocent childhood times. After obtaining all the dragon balls which would grant one a wish, he made his wish and transformed into this mermaid - an epitome where his mind regressed back to the long-forgotten childlike state. This Frieza no longer desires power nor control. No longer interested in training and doing business, all he wants is just to play, with all his heart.