To the witch, the pregnancy was the worst thing that had happened to her, the nausea, the extra kilos, the hormonal changes that always made her cry and laugh out of the sudden...
Therefore, for her, the only one to blame for her condition was the man who had fecundated her, so, to get her revenge, she cursed him and transformed him into a Sea-Horse so he would go through all the suffering she was going through during the pregnancy.
The end.