I have been reading a lot of Pokemon fanfiction lately, and I find that the following female charcters are given these names:
Are these fanmade surnames, or real surnames?
I know this question is answered already, but there's more to this story:
They are fake. Misty's surname is comes from her sisters water show, where they're called the Water flowers
Dawn and Serena's surnames come from their manga counterparts in Pokemon adventures
And mays just comes from in the games she is may birch, and birch and maple are two types of trees.
Most likely Fan Made. There is no saying of those names in the anime(that's what I remember). Also, it's a fan fiction so people are going to make some stuff up to add to it.